NCIRD Vaxbot

On June 30, 2022, CDC announced that Director Rochelle P. Walensky adopted the Decision memo approving the ACIP vote for a preferential recommendation for the use of higher dose (including high dose and recombinant) or adjuvanted flu vaccines over standard-dose unadjuvanted flu vaccines for adults 65 years and older. CDC’s full recommendations for the use of flu vaccines during 2022-2023 will appear in a forthcoming Morbidity and Mortality Weekly Report. Edits to this page are also forthcoming. More information can be found online: CDC Director Adopts Preference for Specific Flu Vaccines for Seniors

About Influenza (Flu)

Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu [vaccine](/flu/prevent/keyfacts.htm) each year. This is new content for this question.

Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose or possibly their eyes.

A 2018 CDC study published in Clinical Infectious Diseases looked at the percentage of the U.S. population who were sickened by flu using two different methods and compared the findings. Both methods had similar findings, which suggested that on average, about 8% of the U.S. population gets sick from flu each season, with a range of between 3% and 11%, depending on the season.

The same CID study found that children are most likely to get sick from flu and that people 65 and older are least likely to get sick from influenza. Median incidence values (or attack rate) by age group were 9.3% for children 0-17 years, 8.8% for adults 18-64 years, and 3.9% for adults 65 years and older. This means that children younger than 18 are more than twice as likely to develop a symptomatic flu infection than adults 65 and older.

Influenza virus infection is so common that the number of people infected each season can only be estimated. These statistical estimations are based on CDC-measured flu hospitalization rates that are adjusted to produce an estimate of the total number of influenza infections in the United States for a given flu season.

The estimates for the number of infections are then divided by the census population to estimate the seasonal incidence (or attack rate) of influenza.

Yes. The proportion of people who get sick from flu varies. A paper published in CID found that between 3% and 11% of the U.S. population gets infected and develops flu symptoms each year. The 3% estimate is from the 2011-2012 season, which was an H1N1-predominant season classified as being of low severity. The estimated incidence of flu illness during two seasons was around 11%; 2012-2013 was an H3N2-predominant season classified as being of moderate severity, while 2014-2015 was an H3N2 predominant season classified as being of high severity.

In the United States, flu season occurs in the fall and winter. While influenza viruses circulate year-round, most of the time flu activity peaks between December and February, but activity can last as late as May. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from season to season. CDC collects, compiles, and analyzes information on influenza activity year-round in the United States and produces FluView, a weekly surveillance report, and FluView Interactive, which allows for more in-depth exploration of influenza surveillance data. The Weekly U.S. Influenza Summary Update is updated each week from October through May.

The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from season to season. CDC collects, compiles, and analyzes information on influenza activity year-round in the United States and produces FluView, a weekly surveillance report, and FluView Interactive, which allows for more in-depth exploration of influenza surveillance data. The Weekly U.S. Influenza Summary Update is updated each week from October through May.  The U.S. influenza surveillance system is a collaborative effort between CDC and its many partners in state and local health departments, public health and clinical laboratories, vital statistics offices, health care providers, and clinics and emergency departments. Information in five categories is collected from eight different data sources that allow CDC to:

  • Find out when and where influenza activity is occurring
  • Track influenza-related illness
  • Determine what influenza viruses are circulating
  • Detect changes in influenza viruses
  • Measure the impact influenza is having on hospitalizations and deaths in the United States

These surveillance components allow CDC to determine when and where influenza activity is occurring, determine what types of influenza viruses are circulating, detect changes in the influenza viruses collected and analyzed, track patterns of influenza-related illness, and measure the impact of influenza in the United States. All influenza activity reporting by states, laboratories, and health care providers is voluntary. For more information about CDC’s influenza surveillance activities, see the Overview of Influenza Surveillance in the United States.

Influenza surveillance data collection is based on a reporting week that starts on Sunday and ends on the following Saturday of each week. Each surveillance participant is requested to summarize the weekly data and submit it to CDC by the following Tuesday afternoon. The data are then downloaded, compiled, and analyzed at CDC. The data are used to update FluView and FluView Interactive on the following Friday.

In addition to flu viruses, several other respiratory viruses also circulate during the flu season and can cause symptoms and illness similar to those seen with flu infection. These respiratory viruses include rhinovirus (one cause of the “common cold”) and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children as well as a leading cause of death from respiratory illness in those aged 65 years and older.

Flu and the common cold are both respiratory illnesses but they are caused by different viruses. Because these two types of illnesses have similar symptoms, it can be difficult to tell the difference between them based on symptoms alone. In general, flu is worse than the common cold, and symptoms are more intense. Colds are usually milder than flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems, such as pneumonia, bacterial infections, or hospitalizations. Flu can have very serious associated complications.

The symptoms of flu can include fever or feeling feverish/chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches and fatigue (tiredness). Cold symptoms are usually milder than the symptoms of flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems.

The timing of flu is difficult to predict and can vary in different parts of the country and from season to season.

Flu viruses are constantly changing so it’s not unusual for new flu viruses to appear each year. More information about how flu viruses change is available.

Influenza (Flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses. COVID-19 is caused by infection with a new coronavirus (called SARS-CoV-2) and flu is caused by infection with influenza viruses. Because some of the symptoms of flu and COVID-19 are similar, it may be hard to tell the difference between them based on symptoms alone, and testing may be needed to help confirm a diagnosis. Flu and COVID-19 share many characteristics, but there are some key differences between the two.

While more is learned every day, there is still a lot that is unknown about COVID-19 and the virus that causes it. This page compares COVID-19 and flu, given the best available information to date.

While it’s not possible to say with certainty what will happen in the fall and winter, CDC believes it’s likely that flu viruses and the virus that causes COVID-19 will both be spreading. In this context, getting a flu vaccine will be more important than ever. CDC recommends that all people 6 months and older get a yearly flu vaccine.

Flu and COVID-19 can both result in serious illness, including illness resulting in hospitalization or death. While there is still much to learn about COVID-19, at this time, it does seem as if COVID-19 is more deadly than seasonal influenza; however, it is too early to draw any conclusions from the current data. This may change as we learn more about the number of people who are infected who have mild illnesses.

To address the importance of influenza vaccination, especially during the COVID-19 pandemic, CDC will maximize flu vaccination by increasing availability of vaccine, including purchasing an additional 2 million doses of pediatric flu vaccine and 9.3 million doses of adult flu vaccine, by emphasizing the importance of flu vaccination for the entire flu season, and by conducting targeted communication outreach to specific groups who are at higher risk for complications from flu. These same groups are often at higher risk for COVID-19 too, so protecting them from influenza is important to decrease their risk of co-infection. Communication strategies for providers and the public will include:

  • Educational outreach activities by CDC, including social media, press conferences, web page spotlights, radio media tours, op-eds, and other publications,
  • A digital campaign to educate the general public and people with who are at increased risk from influenza and COVID-19 complications,
  • Special educational efforts to inform the general population, people with underlying health conditions, and African American and Hispanic audiences about the importance of flu vaccination, and
  • Updated vaccination websites for the public and providers that highlight the safety precautions being implemented in healthcare facilities during the pandemic. ​

Yes. It is possible to have flu, as well as other respiratory illnesses, and COVID-19 at the same time. Health experts are still studying how common this can be.

Some of the symptoms of flu and COVID-19 are similar, making it hard to tell the difference between them based on symptoms alone. Diagnostic testing can help determine if you are sick with flu or COVID-19.

Yes. CDC has developed a test that will check for A and B type seasonal flu viruses and SARS CoV-2, the virus that causes COVID-19. This test will be used by U.S. public health laboratories. Testing for these viruses at the same time will give public health officials important information about how flu and COVID-19 are spreading and what prevention steps should be taken. The test will also help public health laboratories save time and testing materials, and to possibly return test results faster.

The Food and Drug Administration (FDA) has given CDC an Emergency Use Authorization for this new test. Initial test kits were sent to public health laboratories in early August 2020. CDC will continue to manufacture and distribute these kits.

More information for laboratories is available.

No. This new test is designed for use at CDC-supported public health laboratories at state and local levels, where it will supplement and streamline surveillance for flu and COVID-19. The use of this specialized test will be focused on public health surveillance efforts and will not replace any COVID-19 tests currently used in commercial laboratories, hospitals, clinics, and other healthcare settings.

CDC’s first viral test for SARS-CoV-2 (the CDC 2019-nCoV Real-Time RT-PCR Diagnostic Panel (ER-34)) will still be available for qualified laboratories to order through the International Reagent Resource (IRR) external icon. The new multiplex assay can also be ordered through the IRR. Check the IRR website for details.

For additional questions, please visit: Clinical Questions about COVID-19: Questions and Answers: Testing, Diagnosis, and Notification

There is no evidence that getting a flu vaccine increases the risk of getting COVID-19. There are many benefits from flu vaccination and preventing flu is always important, but in the context of the COVID-19 pandemic, it’s even more important to do everything possible to reduce illnesses and preserve scarce health care resources. More Information is available.

Yes. Getting a flu vaccine is an essential part of protecting your health and your family’s health this season. To protect your health when getting a flu vaccine, follow CDC’s recommendations for running essential errands and doctor visits. Continue to take everyday preventive actions.

You can safely get a flu vaccine at multiple locations including your doctor’s office, health departments, and pharmacies. You can use VaccineFinder.orgexternal icon to find where flu vaccines are available near you. When going to get a flu vaccine, be sure to practice everyday preventive actions.

Ask your doctor, pharmacist, or health department if they are following CDC’s vaccination pandemic guidance. Any vaccination location following CDC’s guidance should be a safe place for you to get a flu vaccine.

Efforts to reduce the spread of COVID-19, such as stay-at-home and shelter-in-place orders, have led to decreased use of routine preventive medical services, including immunization services. Ensuring that people continue or start getting routine vaccinations during the COVID-19 pandemic is essential for protecting people and communities from vaccine-preventable diseases and outbreaks, including flu. Routine vaccination prevents illnesses that lead to unnecessary medical visits and hospitalizations, which further strain the healthcare system.

For the upcoming flu season, flu vaccination will be very important to reduce flu because it can help reduce the overall impact of respiratory illnesses on the population and thus lessen the resulting burden on the healthcare system during the COVID-19 pandemic.

A flu vaccine may also provide several individual health benefits, including keeping you from getting sick with flu, reducing the severity of your illness if you do get flu and reducing your risk of a flu-associated hospitalization.

Annual flu vaccination is recommended for everyone 6 months of age and older, with rare exceptions, because it is an effective way to decrease flu illnesses, hospitalizations, and deaths.

During the COVID-19 pandemic, reducing the overall burden of respiratory illnesses is important to protect vulnerable populations at risk for severe illness, the healthcare system, and other critical infrastructure. Thus, healthcare providers should use every opportunity during the influenza vaccination season to administer influenza vaccines to all eligible persons, including;

  • Essential workers: Including healthcare personnel (including nursing home, long-term care facility, and pharmacy staff) and other critical infrastructure workforce
  • Persons at increased risk for severe illness from COVID-19: Including adults aged 65 years and older, residents in a nursing home or long-term care facility, and persons of all ages with certain underlying medical conditions. Severe illness from COVID-19 has been observed to disproportionately affect members of certain racial/ethnic minority groups
  • Persons at increased risk for serious influenza complications: Including infants and young children, children with neurologic conditions, pregnant women, adults aged 65 years and older, and other persons with certain underlying medical conditions

No. Vaccination should be deferred (postponed) for people with suspected or confirmed COVID-19, regardless of whether they have symptoms, until they have met the criteria to discontinue their isolation. While mild illness is not a contraindication to flu vaccination, vaccination visits for these people should be postponed to avoid exposing healthcare personnel and other patients to the virus that causes COVID-19. When scheduling or confirming appointments for vaccination, patients should be instructed to notify the provider’s office or clinic in advance if they currently have or develop any symptoms of COVID-19.

Additionally, a prior infection with suspected or confirmed COVID-19 or flu does not protect someone from future flu infections. The best way to prevent seasonal flu is to get vaccinated every year.

Ask your doctor, pharmacist, or health department if they are following CDC’s vaccination pandemic guidance. Any vaccination location following CDC’s guidance should be a safe place for you to get a flu vaccine.

CDC has released Interim Guidance for Immunization Services During the COVID-19 Pandemic. This guidance is intended to help immunization providers in a variety of clinical and alternative settings with the safe administration of vaccines during the COVID-19 pandemic. This guidance will be continually reassessed and updated based on the evolving epidemiology of COVID-19 in the United States. Healthcare providers who give vaccines should also consult guidance from state, local, tribal, and territorial health officials.

The potential for asymptomatic spread of the virus that causes COVID-19 underscores the importance of applying infection prevention practices to encounters with all patients, including physical distancing (at least 6 feet) when possible, respiratory and hand hygiene, surface decontamination, and source control while in a healthcare facility. Immunization providers should refer to the guidance developed to prevent the spread of COVID-19 in healthcare settings, including outpatient and ambulatory care settings.

To help ensure the safe delivery of care during vaccination visits, providers should:

  • Minimize chances for exposures, including steps such as these:
    • Screen patients for symptoms of COVID-19 and contact with persons with possible COVID-19 prior to and upon their arrival at the facility, and isolate symptomatic patients as soon as possible.
    • Limit and monitor points of entry to the facility and install barriers, such as clear plastic sneeze guards, to limit physical contact with patients at triage.
    • Implement policies for adults and children over the age of 2 years to wear cloth face coverings (if tolerated).
    • Ensure patients practice respiratory hygiene, cough etiquette, and hand hygiene.
  • Ensure all staff adhere to the following infection prevention and control procedures:
    • Follow Standard Precautions, which include guidance for hand hygiene and cleaning the environment between patients.
    • Wear a medical face mask at all times.
    • Use eye protection based on level of community transmission of the virus that causes COVID-19:
      • Moderate-to-substantial transmission: Healthcare providers should wear eye protection given the increased likelihood of encountering asymptomatic COVID-19 patients.
      • Minimal-to-no transmission: Universal eye protection is considered optional, unless otherwise indicated as a part of Standard Precautions.
  • Consider these additional steps during vaccine administration:
    • Intranasal or oral vaccines:
      • Healthcare providers should wear gloves when giving intranasal or oral vaccines because of the increased likelihood of coming into contact with a patient’s mucous membranes and body fluids. They should change their gloves and wash their hands between patients.
      • Giving these vaccines is not considered an aerosol-generating procedure and thus, the use of an N95 or higher-level respirator is not recommended.
    • Intramuscular or subcutaneous vaccines:
  • For patients (sick or well) presenting for care or routine visits, ensure physical distancing by implementing strategies, such as:
    • Separating sick from well patients by scheduling these visits during different times of the day (e.g., well visits in the morning and sick visits in the afternoon), placing patients with sick visits in different areas of the facility, or scheduling patients with sick visits in a different location from well visits (when space is available).
    • Reduce crowding in waiting areas by asking patients to remain outside (e.g., stay in their vehicles, if applicable) until they are called into the facility for their appointment.
    • Ensure that physical distancing measures, with separation of at least 6 feet between patients and visitors, are maintained during all aspects of the visit, including check-in, checkout, screening procedures, and postvaccination monitoring. Use strategies such as physical barriers, signs, ropes, and floor markings.
    • Use electronic communications as much as possible (e.g., filling out needed paperwork online in advance) to minimize patients’ time in the office as well as their sharing of materials (e.g., clipboards, pens).

Yes. Guidance has been developed for giving vaccines at pharmaciestemporary, off-site, or satellite clinics, and large-scale influenza clinics. Other approaches to vaccination during the COVID-19 pandemic may include drive-through immunization services at fixed sites, curbside clinics, mobile outreach units, and home visits.

The general principles outlined for healthcare facilities should also be applied to alternative vaccination sites, with additional precautions for physical distancing that are particularly relevant for large-scale clinics, such as:

  • Providing specific appointment times or other strategies to manage patient flow and avoid crowding.
  • Ensuring sufficient staff and resources to help move patients through the clinic as quickly as possible.
  • Limiting the overall number of patients at any given time, particularly for populations at higher risk for severe illness from COVID-19.
  • Setting up a one-way flow through the site and using signs, ropes, or other measures to direct patient traffic and ensure physical distancing between patients.
  • Arranging a separate vaccination area or separate hours for persons at increased risk for severe illness from COVID-19, such as older adults and persons with underlying medical conditions, when feasible.
  • Selecting a space large enough to ensure a minimum distance of 6 feet between patients in line or in waiting areas for vaccination, between vaccination stations, and in postvaccination monitoring areas (the Advisory Committee on Immunization Practices recommends that providers consider observing patients for 15 minutes after vaccination to decrease the risk for injury should they faint).

Yes. CDC has resources to help with vaccine planning during the COVID-19 pandemic. Ask your doctor, pharmacist, or health department if they are following CDC’s vaccination pandemic guidance.  And protect yourself by practicing everyday preventive actions.

Diagnosis

Your respiratory illness might be the flu if you have fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and/or fatigue. Some people may have vomiting and diarrhea, though this is more common in children. People may be sick with flu and have respiratory symptoms without a fever. Flu viruses usually cause the most illness during the colder months of the year. However, influenza can also occur outside of the typical flu season. In addition, other viruses can also cause respiratory illness similar to flu. So, it is impossible to tell for sure if you have flu based on symptoms alone. If your doctor needs to know for sure whether you are sick with flu, there are laboratory tests that can be done.

A number of flu tests are available to detect influenza viruses in respiratory specimens. The most common are called “rapid influenza diagnostic tests (RIDTs).” RIDTs work by detecting the parts of the virus (antigens) that stimulate an immune response. These tests can provide results within approximately 10-15 minutes but are not as accurate as other flu tests. Therefore, you could still have flu, even though your rapid test result is negative. Other flu tests are called “rapid molecular assays” that detect genetic material of the virus. Rapid molecular assays produce results in 15-20 minutes and are more accurate than RIDTs. In addition, there are several more accurate and sensitive flu tests available that must be performed in specialized laboratories, such as those found in hospitals or state public health laboratories. All of these tests require that a health care provider swipe the inside of your nose or the back of your throat with a swab and then send the swab for testing. Results may take one to several hours.

During an influenza outbreak, a positive rapid flu test is likely to indicate influenza infection. However, rapid tests vary in their ability to detect flu viruses, depending on the type of rapid test used, and on the type of flu viruses circulating. Also, rapid tests appear to be better at detecting flu in children than adults. This variation in ability to detect viruses can result in some people who are infected with flu having a negative rapid test result. (This situation is called a false negative test result.) Despite a negative rapid test result, your health care provider may diagnose you with flu based on your symptoms and their clinical judgment.

Your respiratory illness might be flu if you have fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and/or fatigue. Some people may have vomiting and diarrhea, though this is more common in children. People may be sick with flu and have respiratory symptoms without a fever. Flu viruses usually cause the most illness during the colder months of the year. However, influenza can also occur outside of the typical flu season. In addition, other viruses can also cause respiratory illness similar to flu. So, it is impossible to tell for sure if you have flu based on symptoms alone. If your doctor needs to know for sure whether you are sick with flu, there are laboratory tests that can be done.

Please visit diagnosing flu for more information.

Flu Antiviral Drugs

Yes. There are prescription medications called “antiviral drugs” that can be used to treat flu illness. CDC recommends prompt treatment for people who have flu infection or suspected flu infection and who are at high risk of serious flu complications, such as people with asthma, diabetes (including gestational diabetes), or heart disease.

Antiviral drugs are prescription medicines (pills, liquid, an inhaled powder, or an intravenous solution) that fight against flu viruses in your body. Antiviral drugs are not sold over-the-counter. You can only get them if you have a prescription from a health care provider. Antiviral drugs are different from antibiotics, which fight against bacterial infections.

Yes. Antiviral drugs are not a substitute for getting a flu vaccine. While flu vaccine can vary in how well it works, a flu vaccine is best way to help prevent seasonal flu and its potentially serious complications. Antiviral drugs are a second line of defense that can be used to **treat** flu (including seasonal flu and variant flu viruses) if you get sick.

Antiviral treatment works best when started soon after flu illness begins. When treatment is started within two days of becoming sick with flu symptoms, antiviral drugs can lessen fever and flu symptoms, and shorten the time you are sick by about one day. They also may reduce the risk of complications such as ear infections in children, respiratory complications requiring antibiotics, and hospitalization in adults.  For people at high risk of serious flu complications, early treatment with an antiviral drug can mean having milder illness instead of more severe illness that might require a hospital stay.  For adults hospitalized with flu illness, some studies have reported that early antiviral treatment can reduce their risk of death.

Studies show that flu antiviral drugs work best for treatment when they are started within two days of getting sick. However, starting them later can still be beneficial, especially if the sick person is at high risk of serious flu complications or is in the hospital with more severe illness.  Follow instructions for taking these drugs.

There are four FDA-approved antiviral drugs recommended by CDC to treat flu this season.

  • oseltamivir phosphate (available as a generic version or under the trade name Tamiflu®),
  • zanamivir (trade name Relenza®)
  • peramivir (trade name Rapivab®), and
  • baloxavir marboxil (trade name Xofluza®).

Generic oseltamivirexternal icon and Tamiflu® are available as a pill or liquid suspension and are FDA approved for early treatment of flu in people 14 days and older. Zanamivir is a powder that is inhaled and approved for early treatment of flu in people 7 years and older. (Note: Zanamivir (trade name Relenza®) is administered using an inhaler device and is not recommended for people with breathing problems like asthma or COPD.) Peramivir is given intravenously by a health care provider and is approved for early treatment of flu in people 2 years and older. Baloxavir is a pill given as a single dose by mouth and is approved for early treatment of flu in people 12 years and older. (Note: Baloxavir (trade name Xofluza®) is not recommended for pregnant women, breastfeeding mothers, outpatients with complicated or progressive illness, or hospitalized patients because there is no information about use of baloxavir in these patients.)

Duration of treatment varies depending on the antiviral drug prescribed. Oseltamivir and zanamivir are usually prescribed to be taken twice daily for 5 days, although people hospitalized with flu may need antiviral treatment for longer than 5 days. Peramivir is given one time intravenously over a period of 15 to 30 minutes. Baloxavir is given as a single oral dose.

Side effects vary for each medication. The most common side effects for oseltamivir are nausea and vomiting. Zanamivir can cause bronchospasm, and peramivir can cause diarrhea. Other less common side effects also have been reported. Your health care provider can give you more information about these drugs or you can check the Food and Drug Administration (FDA) websiteexternal icon for specific information about antiviral drugs, including the manufacturer’s package insert.

Yes. Oseltamivir is recommended by CDC and the American Academy of Pediatrics (AAP) for early treatment of flu in people of any age, and for the prevention of flu (i.e., prophylaxis) in people 3 months and older. Zanamivir is recommended for early treatment of flu in people 7 years and older, and for the prevention of flu in people 5 years and older. Peramivir is recommended for early treatment in people 2 years and older. Baloxavir is recommended for early treatment of flu in people 12 years and older.

If your child’s health care provider prescribes oseltamivir capsules for your child and your child cannot swallow capsules, the prescribed capsules may be opened, mixed with a thick sweetened liquid, and given that way. Learn more here.

Yes. Oral oseltamivir is recommended for treatment of pregnant women with flu because compared to other recommended antiviral medications, it has the most studies available to suggest that it is safe and beneficial during pregnancy. Baloxavir is not recommended for pregnant women or breastfeeding mothers, as there are no available efficacy or safety data.

It’s very important that flu antiviral drugs are started as soon as possible to treat hospitalized flu patients, people who are very sick with flu but who do not need to be hospitalized, and people who are at high risk of serious flu complications based on their age or health if they develop flu symptoms. Although other people with mild illness who are not at high risk of flu complications may also be treated early with antiviral drugs by their doctor, most people who are otherwise healthy and get flu do not need to be treated with antiviral drugs.

What to do if You Get Sick

Most people with the flu have mild illness and do not need medical care or antiviral drugs. If you get sick with flu symptoms, in most cases, you should stay home and avoid contact with other people except to get medical care.

If, however, you have symptoms of flu and are in a high risk group, or are very sick or worried about your illness, contact your health care provider (doctor, physician assistant, etc.).

Certain people are at high risk of serious flu-related complications (including young children, people 65 and older, pregnant women and people with certain medical conditions). This is true both for seasonal flu and novel flu virus infections. (For a full list of people at high risk of flu-related complications, see People at High Risk of Developing Flu–Related Complications). If you are in a high risk group and develop flu symptoms, it’s best for you to contact your doctor early in your illness. Remind them about your high risk status for flu. CDC recommends that people at high risk for complications should get antiviral treatment as early as possible, because benefit is greatest if treatment is started within 2 days after illness onset.

No. The emergency room should be used for people who are very sick. You should not go to the emergency room if you are only mildly ill.

If you have the emergency warning signs of flu sickness, you should go to the emergency room. If you get sick with flu symptoms and are at high risk of flu complications or you are concerned about your illness, call your health care provider for advice. If you go to the emergency room and you are not sick with the flu, you may catch it from people who do have it.

People experiencing these warning signs should obtain medical care right away.

In children

  • Fast breathing or trouble breathing
  • Bluish lips or face
  • Ribs pulling in with each breath
  • Chest pain
  • Severe muscle pain (child refuses to walk)
  • Dehydration (no urine for 8 hours, dry mouth, no tears when crying)
  • Not alert or interacting when awake
  • Seizures
  • Fever above 104°F
  • In children less than 12 weeks, any fever
  • Fever or cough that improve but then return or worsen
  • Worsening of chronic medical conditions

In adults

  • Difficulty breathing or shortness of breath
  • Persistent pain or pressure in the chest or abdomen
  • Persistent dizziness, confusion, inability to arouse
  • Seizures
  • Not urinating
  • Severe muscle pain
  • Severe weakness or unsteadiness
  • Fever or cough that improve but then return or worsen
  • Worsening of chronic medical conditions

These lists are not all inclusive. Please consult your medical provider for any other symptom that is severe or concerning.

Yes. There are drugs your doctor may prescribe for treating the flu called “antivirals.” These drugs can make you better faster and may also prevent serious complications. See Treatment – Antiviral Drugs for more information.

CDC recommends that you stay home for at least 24 hours after your fever is gone except to get medical care or other necessities. Your fever should be gone without the need to use a fever-reducing medicine, such as Tylenol®. Until then, you should stay home from work, school, travel, shopping, social events, and public gatherings.

CDC also recommends that children and teenagers (anyone aged 18 years and younger) who have flu or are suspected to have flu should not be given Aspirin (acetylsalicylic acid) or any salicylate containing products (e.g. Pepto Bismol); this can cause a rare, very serious complication called Reye’s syndrome. More information about Reye’s syndrome can be found hereexternal icon.

Stay away from others as much as possible to keep from infecting them. If you must leave home, for example to get medical care, wear a facemask if you have one, or cover coughs and sneezes with a tissue. Wash your hands often to keep from spreading flu to others.

Vaccine / Flu Shot

An influenza shot is a vaccine given with a needle, usually in the arm. Seasonal influenza shots protect against the three or four influenza viruses that research indicates will be most common during the season.

The composition of the influenza virus has been updated for the 2019-2020 influenza season.

Everyone 6 months and older should get a flu vaccine every season with rare exceptions. Vaccination is particularly important for people who are at high risk of serious complications from influenza. People at High Risk of Developing Flu-Related Complications has a full list of age and health factors that confer increased risk.

Flu vaccination has important benefits. It can reduce flu illnesses, doctors’ visits, and missed work and school due to flu, as well as prevent flu-related hospitalizations and deaths.

Different flu vaccines are approved for use in different groups of people.

There are many vaccine options to choose from.  CDC does not recommend one flu vaccine over another. The most important thing is for all people 6 months of age and older to get a flu vaccine every year.

If you have questions about which flu vaccine to get, talk to your doctor or other health care professional. More information is available at Who Should Get Vaccinated.

Flu vaccines are offered in many doctor’s offices, clinics, health departments, pharmacies and college health centers, as well as by many employers, and even in some schools.

Even if you don’t have a regular doctor or nurse, you can get a flu vaccine somewhere else, like a health department, pharmacy, urgent care clinic, and often your school, college health center, or workplace.

Visit the Vaccine Finder to locate where you can get a flu vaccine.

Influenza is a potentially serious disease that can lead to hospitalization and sometimes even death. Every flu season is different, and influenza infection can affect people differently, but millions of people get flu every year, hundreds of thousands of people are hospitalized and thousands to tens of thousands of people die from flu-related causes every year. An annual seasonal flu vaccine is the best way to help protect against flu. Vaccination has been shown to have many benefits including reducing the risk of flu illnesses, hospitalizations and even the risk of flu-related death in children.

A flu vaccine is needed every season for two reasons. First, a person’s immune protection from vaccination declines over time, so an annual vaccine is needed for optimal protection. Second, because flu viruses are constantly changing, flu vaccines may be updated from one season to the next to protect against the viruses that research suggests may be most common during the upcoming flu season. For the best protection, everyone 6 months and older should get vaccinated annually.

Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection with the viruses that are used to make the vaccine.

The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season. Most flu vaccines in the United States protect against four different flu viruses (“quadrivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses. There are also some flu vaccines that protect against three different flu viruses (“trivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and one influenza B virus. Two of the trivalent vaccines are designed specifically for people 65 and older to create a stronger immune response.

You should get a flu vaccine before flu viruses begins spreading in your community, since it takes about two weeks after vaccination for antibodies to develop in the body and provide protection against flu. Make plans to get vaccinated early in fall, before flu season begins. CDC recommends that people get a flu vaccine by the end of October. However, getting vaccinated early (for example, in July or August) is likely to be associated with reduced protection against flu infection later in the flu season, particularly among older adults. Vaccination should continue to be offered throughout the flu season, even into January or later. Children who need two doses of vaccine to be protected should start the vaccination process sooner, because the two doses must be given at least four weeks apart.

Different influenza (flu) vaccines are approved for use in different age groups. In addition, some vaccines are not recommended for certain groups of people. Factors that can determine a person’s suitability for vaccination, or vaccination with a particular vaccine, include a person’s age, health (current and past) and any allergies to flu vaccine or its components. For more information, visit Who Should and Who Should NOT get a Flu Vaccine.

Studies have shown the small amount of thimerosal in vaccines does not cause harm. There is thimerosal-free flu vaccine available for people who want to avoid thimerosal. More information about thimerosal can be found at Thimerosal and Flu Vaccines

Yes. Breastfeeding women should get the flu vaccine to protect themselves from flu. Getting vaccinated reduces mothers’ risk of getting sick and of passing the flu on to their babies, thus protecting their babies from flu also. This is especially important for children younger than 6 months old since they are too young to receive flu vaccine themselves.

No, flu vaccines cannot cause flu illness. Flu vaccines given with a needle (i.e., flu shots) are made with either inactivated (killed) viruses, or with only a single protein from the flu virus.  The nasal spray vaccine contains live viruses that are attenuated (weakened) so that they will not cause illness.

There are several reasons why someone might get flu symptoms, even after they have been vaccinated against flu.

  1. One reason is that some people can become ill from other respiratory viruses besides flu such as rhinoviruses, which are associated with the common cold, cause symptoms similar to flu, and also spread and cause illness during the flu season. The flu vaccine only protects against flu, not other illnesses.
  2. Another explanation is that it is possible to be exposed to flu viruses, which cause flu, shortly before getting vaccinated or during the two-week period after vaccination that it takes the body to develop immune protection. This exposure may result in a person becoming ill with flu before protection from vaccination takes effect.
  3. A third reason why some people may experience flu  symptoms despite getting vaccinated is that they may have been exposed to a flu virus that is very different from the viruses the vaccine is designed to protect against. The ability of a flu vaccine to protect a person depends largely on the similarity or “match” between the viruses selected to make the vaccine and those spreading and causing illness. There are many different flu viruses that spread and cause illness among people. For more information, see Influenza (Flu) Viruses.
  4. The final explanation for experiencing flu symptoms after vaccination is that flu vaccines vary in how well they work and some people who get vaccinated still get sick. When that happens, though vaccination has been shown in several studies to reduce severity of illness in those people who get vaccinated but still get sick.

No. Flu can be a serious disease, particularly among young children, older adults, and people with certain chronic health conditions, such as asthma, heart disease or diabetes. Any flu infection can carry a risk of serious complications, hospitalization or death, even among otherwise healthy children and adults. Therefore, getting vaccinated is a safer choice than risking illness to obtain immune protection.

It’s possible to get sick with flu even if you have been vaccinated (although you won’t know for sure unless you get a flu test). This is possible for the following reasons:

  1. You may be exposed to an influenza virus shortly before getting vaccinated or during the period that it takes the body to gain protection after getting vaccinated. This exposure may result in you becoming ill with influenza before the vaccine begins to protect you. (Antibodies that provide protection develop in the body about 2 weeks after vaccination.)
  2. You may be exposed to an influenza virus that is different from what is used to make seasonal influenza vaccine. There are many different influenza viruses that circulate every year. An influenza vaccine is made to protect against the three or four influenza viruses that research suggests will be most common.
  3. Unfortunately, some people can become infected with an influenza virus that the influenza vaccine is designed to protect against, despite getting vaccinated. Protection provided by influenza vaccination can vary widely, based in part on health and age factors of the person getting vaccinated. In general, an influenza vaccine works best among healthy younger adults and older children. Some older people and people with certain chronic illnesses may develop less immunity after vaccination. Influenza vaccination is not a perfect tool, but it is the best way to protect against influenza infection.

More Information: How Well Does the Seasonal Flu Vaccine Work?

In adults, studies have not shown a benefit from getting more than one dose of vaccine during the same influenza season, even among elderly persons with weakened immune systems. Except for children getting vaccinated for the first time, only one dose of flu vaccine is recommended each season.

How long you are immune or your “duration of immunity” is discussed in the ACIP recommendations. While delaying getting of vaccine until later in the fall may lead to higher levels of immunity during winter months, this should be balanced against possible risks, such as missed opportunities to receive vaccine and difficulties associated with vaccinating a large number of people within a shorter time period.

No. Vaccination can still be beneficial as long as flu viruses are circulating. If you have not been vaccinated by Thanksgiving (or the end of November), it can still be protective to get vaccinated in December or later. Flu is unpredictable and seasons can vary. Seasonal flu disease usually peaks between December and March most years, but disease can occur as late as May.

No. There is no recommendation for pregnant women or people with pre-existing medical conditions to seek special permission or secure written consent from their doctor for vaccination if they get vaccinated at a worksite clinic, pharmacy or other location outside of their physician’s office. With rare exception, CDC recommends an annual flu vaccine for everyone 6 months of age and older, including pregnant women and people with medical conditions.

A variety of flu vaccine products are available (Table 1). Vaccine providers should be aware of the approved age indications of the vaccine they are using and of any contraindications or precautions. Providers also should appropriately screen all people getting vaccinated for allergies to vaccine components or other contraindications. People who have previously had a severe allergic reaction to influenza vaccine or any of its ingredients should generally not be vaccinated.

There are some people who should not get a flu vaccine without first speaking with their doctor. These include:

  • People who have a moderate-to-severe illness with or without a fever (they should wait until they recover to get vaccinated), and
  • People with a history of Guillain-Barré Syndrome (a severe paralytic illness, also called GBS) that occurred after receiving influenza vaccine and who are not at risk for severe illness from influenza should generally not receive vaccine. Tell your doctor if you ever had Guillain-Barré Syndrome. Your doctor will help you decide whether the vaccine is recommended for you.

Pregnant women or people with pre-existing medical conditions who get vaccinated should get a flu shot.

If a person is vaccinated by someone other than their primary health care provider, the vaccinating provider should ensure that the patient and, if possible, the patient’s medical provider have documentation of vaccination.

For a complete list of people who should not get a flu vaccine before speaking with their doctor, please review the influenza Vaccine Information Statement for the inactivated or recombinant flu vaccine or live, intranasal influenza vaccine.

No. Many people use the term “stomach flu” to describe illnesses with nausea, vomiting or diarrhea. These symptoms can be caused by many different viruses, bacteria or even parasites. While vomiting, diarrhea, and being nauseous or “sick to your stomach” can sometimes be related to flu — more commonly in children than adults — these problems are rarely the main symptoms of influenza. Flu is a respiratory disease and not a stomach or intestinal disease.

2018-19 flu vaccination coverage among health care personnel (HCP) was 81.1%, similar to coverage during the past four seasons (77.3% -79.0%).

  • By occupation, flu vaccination coverage was highest among physicians (96.7%), nurses (98.1%), pharmacists (91.5%), and nurse practitioners and physician assistants (91.0%)
  • Flu vaccination coverage was lowest among other clinical health care personnel (85.8%), assistants and aides (72.5%), and nonclinical health care personnel (75.5%).

By work setting, flu vaccination coverage was highest among HCP working in hospitals (95.2%).

  • Flu vaccination coverage continues to be lower among HCP working in long-term care (LTC) settings (67.9%) compared with those working in hospitals and ambulatory settings and physician offices (79.8%).
  • Vaccination coverage was highest (97.7%) among health care personnel working in settings where vaccination was required. Among health care personnel whose employers did not have a requirement for vaccination coverage was higher among those who worked in locations where vaccination was offered at the worksite at no cost for 1 day only (75.6%) or >1 day (83.2%) or who worked in locations where their employer did not provide influenza vaccination on-site at no cost but actively promoted vaccination through other mechanisms (75.6%), compared with that among health care personnel working in locations where employers did not have any vaccination-related requirements or provisions (42.1%).

Influenza Vaccination Coverage Among Health Care Personnel — United States, 2018–19 Influenza Season for more information.

A Toolkit for Long-term Care Employers: Increasing Influenza Vaccination among Healthcare Personnel in Long-term Care Settings
.

The findings of a CDC review of related published literature indicate that influenza vaccination of health care personnel can enhance patient safety. 1,2

CDC conducts science-based investigations, research, and public health surveillance both nationally and internationally. CDC adopts recommendations that are made by the Advisory Committee for Immunization Practices. These recommendations may be considered by state and other Federal agencies when making or enforcing laws. CDC also has infection control recommendations for health care settings. However, CDC does not issue any requirements or mandates for state agencies, health systems, or health care workers regarding infection control practices, including influenza vaccination or the use of masks.

Some employers require certain immunizations. Hospitals, for example, may require some staff to get a flu vaccine or hepatitis B vaccine or take other precautions such as the use of masks.

To find out more about the laws in your state and  to contact your state health department through Public Health Resources: State Health Departments.

State Immunization Laws for Healthcare Workers and Patients and Vaccines and Immunizations: Basics and Common Questions National Center for Immunization and Respiratory Diseases have more information. For more information, updates, and access to free materials to assist with educating staff and patients about the impact of influenza and the benefits of vaccination, visit CDC Seasonal Influenza (Flu) or call the National Immunization Hotline at (800) 232-2522 (English), (800) 232-0233 (Español), or (800) 243-7889 (TTY).

There is no change in CDC’s recommendation on timing of vaccination this flu season. Getting vaccinated in July or August is too early, especially for older people, because of the likelihood of reduced protection against flu infection later in the flu season. September and October are good times to get vaccinated. However, as long as flu viruses are circulating, vaccination should continue, even in January or later.

More information for vaccination timing this year.

How and where people get a flu vaccine may need to change due to the COVID-19 pandemic. CDC works with healthcare providers and state and local health departments to develop contingency plans on how to vaccinate people against flu without increasing their risk of exposure to respiratory germs, like the virus that causes COVID-19, and has released Interim Guidance for Immunization Services During the COVID-19 Pandemic. More information is available Administering Flu Vaccines during the COVID-19 Pandemic.

Some settings that usually provide flu vaccine, like workplaces, may not offer vaccination this upcoming season, because of the challenges with maintaining social distancing. For more information on where you can get a flu vaccine, visit Vaccine Finder. Information on getting a flu vaccine safely this season is available Getting a Flu Vaccine during the COVID-19 Pandemic.

Vaccine Safety Questions & Answers

Flu vaccines have a good safety record. Hundreds of millions of Americans have safely received flu vaccines over the past 50 years, and there has been extensive research supporting the safety of flu vaccines.

A flu vaccine is the first and best way to reduce your chances of getting the flu and spreading it to others. CDC recommends that everyone 6 months of age and older receive a flu vaccine every year.

No, the flu vaccine cannot cause flu. The vaccines either contain inactivated virus, meaning the viruses are no longer infectious, or a particle designed to look like a flu virus to your immune system. While the nasal spray flu vaccine does contain a live virus, the viruses are changed so that they cannot give you the flu.

If you believe you have been injured by a flu vaccine you may be eligible to receive compensation from the federal government for your injuries if certain criteria are met. To learn more visit the National Vaccine Injury Compensation Program website or call 1-800-338-2382.

CDC and the Food and Drug Administration (FDA) closely monitor the safety of vaccines approved for use in the United States. CDC uses two primary systems to monitor the safety of flu vaccines:

  1. Vaccine Adverse Event Reporting System (VAERS): an early warning system that helps CDC and FDA monitor problems following vaccination. Anyone can report possible vaccine side effects to VAERS. Generally, VAERS reports cannot determine if an adverse event was caused by a vaccine, but these reports can help determine if further investigations are needed.
  2. Vaccine Safety Datalink (VSD): A collaboration between CDC and nine health care organizations which allows ongoing monitoring and proactive searches of vaccine-related data.

Persons who are able to eat lightly cooked egg (e.g., scrambled egg) without a reaction are unlikely to be allergic. Egg-allergic persons might tolerate egg in baked products (e.g., bread or cake). Tolerance to egg-containing foods does not exclude the possibility of egg allergy. Egg allergy can be confirmed by a consistent medical history of adverse reactions to eggs and egg-containing foods, plus skin or blood testing for immunoglobulin E directed against egg proteins.

Egg allergy affects about 1.3 % of all children and 0.2 % of all adults.

If you are able to eat lightly cooked egg (e.g., scrambled egg) without reaction, you are unlikely to have an egg allergy and can get any licensed flu vaccine (i.e., any form of IIV, LAIV, or RIV) that is otherwise appropriate for your age and health status.

If you are someone with a history of egg allergy, who has experienced only hives after exposure to egg, you can get any licensed flu vaccine (i.e., any form of IIV, LAIV, or RIV) that is otherwise appropriate for your age and health.

If you are someone who has more serious reactions to eating eggs or egg-containing foods, like angioedema, respiratory distress, lightheadedness, or recurrent emesis; or who required epinephrine or another emergency medical intervention, you can get any licensed flu vaccine (i.e., any form of IIV, LAIV, or RIV) that is otherwise appropriate for your age and health status, but the vaccine should be given in an inpatient or outpatient medical setting (including but not necessarily limited to hospitals, clinics, health departments, and physician offices), under the supervision of a health care provider who is able to recognize and manage severe allergic conditions.

People with egg allergy can receive flu vaccines according to the recommendations above. A person who has previously experienced a severe allergic reaction to flu vaccine, regardless of the component suspected of being responsible for the reaction, should not get a flu vaccine again.

Most flu vaccines today are produced using an egg-based manufacturing process and thus contain a small amount of egg protein called ovalbumin.

For the 2019-2020 flu season, there are two vaccines licensed for use that are manufactured without the use of eggs and are considered egg-free:

  • Flublok Quadrivalent (licensed for use in adults 18 years and older)
  • Flucelvax Quadrivalent (licensed for use in people 4 years and older)

While not all manufacturers disclose the amount of ovalbumin in their vaccines, those that did from 2011–12 through 2014–15 reported maximum amounts of ≤1 µg/0.5 mL dose for flu shots and 0.24 µg/0.2 mL dose for the nasal spray vaccine. Recombinant vaccine (Flublok Quadrivalent) and cell-based vaccine (Flucelvax Quadrivalent) are the only vaccines currently available that are completely egg free.

Yes, allergic reactions can happen, but they occur very rarely with the flu vaccines available in the United States today. Occasional cases of anaphylaxis, a severe life-threatening reaction that involves multiple organ systems and can progress rapidly, in egg-allergic persons have been reported to the Vaccine Adverse Event Reporting System (VAERS) after administration of flu vaccine. Flu vaccines contain various components that may cause allergic reactions, including anaphylaxis. In a Vaccine Safety Datalink study, there were 10 cases of anaphylaxis after more than 7.4 million doses of inactivated flu vaccine, trivalent (IIV3) given without other vaccines, (rate of 1.35 per one million doses). Most of these cases of anaphylaxis were not related to the egg protein present in the vaccine. CDC and the Advisory Committee on Immunization Practices continue to review available data regarding anaphylaxis cases following flu vaccines.

Allergic reactions can begin very soon after vaccination. However, the onset of symptoms is sometimes delayed. In a Vaccine Safety Datalink study of more than 25.1 million doses of vaccines of various types given to children and adults over 3 years, only 33 people had anaphylaxis. Of patients with a documented time to onset of symptoms, eight cases had onset within 30 minutes of vaccination, while in another 21 cases, symptoms were delayed more than 30 minutes following vaccination, including one case with symptom onset on the following day.

Yes. Flu shots have been given to millions of pregnant women over many years with a good safety record. There is a large body of scientific studies that supports the safety of flu vaccine in pregnant women and their babies. CDC continues to gather data on this topic.

Most people who have an allergy to eggs can get vaccinated, with some additional safety measures. A person with severe (life-threatening) allergy to any vaccine component, including egg protein, should not get the shot, even if she is pregnant. Pregnant women should tell the person giving the shots if they have any severe allergies or if they have ever had a severe allergic reaction following a flu shot.

People with egg allergies can receive any licensed, recommended age-appropriate influenza (flu) vaccine (IIV, RIV4, or LAIV4) that is otherwise appropriate.  People who have a history of severe egg allergy (those who have had any symptom other than hives after exposure to egg) should be vaccinated in a medical setting, supervised by a health care provider who is able to recognize and manage severe allergic reactions.

Vaccine Side Effects

The most common side effects experienced by pregnant women, high risk, hiv-positive are the same as those experienced by other people. They are generally mild and include:

  • Soreness, redness, and/or swelling from the shot
  • Fainting
  • Headache
  • Fever
  • Muscle aches
  • Nausea
  • Fatigue

If side effects occur, they usually begin soon after the shot is given and generally last for 1-2 days.

Rarely, flu shots can cause serious problems like severe allergic reactions. Anyone with a severe, life-threatening allergy to any of the vaccine ingredients should not get the shot.

Life-threatening allergic reactions to influenza shots are very rare. Signs of serious allergic reaction can include breathing problems, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness. If they do occur, it is usually within a few minutes to a few hours after receiving the shot. These reactions can occur among persons who are allergic to something that is in the vaccine, such as egg protein or other ingredients. While severe reactions are uncommon, you should let your doctor, nurse, clinic, or pharmacist know if you have a history of allergy or severe reaction to influenza vaccine or any part of influenza vaccine.

There is a small possibility that influenza vaccine could be associated with Guillain-Barré syndrome, generally no more than 1 or 2 cases per million people vaccinated. This is much lower than the risk of severe complications from influenza, which can be prevented by influenza vaccine.

Call a doctor or get to a doctor right away.

Tell your doctor what happened, the date and time it happened, and when you got the flu shot.

Ask your doctor, nurse, or health department to file a Vaccine Adverse Event Reporting System (VAERS) form, or call VAERS at 1-800-822-7967. Reports are welcome from all concerned individuals: patients, parents, health care providers, pharmacists and vaccine manufacturers.

Influenza vaccine side effects are generally mild and go away on their own within a few days. Some side effects that may occur from an influenza shot include soreness, redness, and/or swelling where the shot was given, headache (low grade), fever, nausea, muscle aches, and fatigue. The influenza shot, like other injections, can occasionally cause fainting.

Flu vaccines are not thought to make people more susceptible to other respiratory infections.

A 2012 study that suggested that flu vaccination might make people more susceptible to other respiratory infections. After that study was published, many experts looked into this issue further and conducted additional studies to see if the findings could be replicated. No other studies have found this effect. It’s not clear why this finding was detected in the one study, but the majority of evidence suggests that this is not a common or regular occurrence and that flu vaccination does not, in fact, make people more susceptible to other respiratory infections.

Multiple studies have shown that women who have gotten flu shots during pregnancy have not had a higher risk for spontaneous abortion (miscarriage). One of the largest and strongest studies was conducted in CDC’s Vaccine Safety Datalink (VSD) project. The recently published study covered three flu seasons (2012-13, 2013-14, 2014-15) looking for any increased risk for miscarriage among pregnant women who had received a flu vaccine during their pregnancy. The study found NO increased risk for miscarriage after flu vaccination during pregnancy. This study was conducted in follow-up to a previous smaller study. The prior study examined data from the 2010-2011 and 2011-2012 flu seasons and identified an association between flu vaccination early in pregnancy and an increased risk of spontaneous abortion or miscarriage; particularly among women who had received flu vaccine during the previous flu season. However, the study had several limitations, including small sample size which could have led to imprecise results. This study was the only analysis to show that association; no other studies had found an increased risk of SAB following flu vaccination At this time, the Advisory Committee on Immunization Practices (ACIP), the American College of Obstetricians and Gynecologists (ACOG) and CDC continue to recommend that pregnant women get a flu vaccine during any trimester of their pregnancy because flu poses a danger to pregnant women and a flu vaccine can prevent serious illness, including hospitalization, in pregnant women.

A fact sheet with more information on this topic is available. Any pregnant woman who has questions about vaccines should talk to her doctor.

The word “febrile” refers to having a fever. A seizure is a convulsion or fit of uncontrolled body movements. A “febrile seizure” refers to a seizure/convulsion associated with a fever in a child. Febrile seizures usually last around one or two minutes and can occur with any illness that causes fever, such as colds, influenza, or ear infection. They are most common with fevers of 102°F (38.9°C) or higher, but they can also happen at lower body temperatures or when a fever is going down. A person experiencing a febrile seizure may lose consciousness.

Most febrile seizures happen in children between the ages of 6 months and 5 years. Up to 5% of young children will have at least one febrile seizure. The most common age range for children to have febrile seizures is 14–18 months.

About 1 in 3 children who have one febrile seizure will have more febrile seizures during childhood. If a member of a child’s immediate family (a brother, sister, or parent) has had febrile seizures, that child is more likely to have a febrile seizure.

Febrile seizures can be frightening, but nearly all children who have a febrile seizure recover quickly, are healthy afterwards, and do not have any permanent neurological damage. Febrile seizures do not make children more likely to develop epilepsy or any other seizure disorder.

Vaccines can cause fevers, but febrile seizures are rare after vaccination. Importantly, getting sick with the flu also can cause febrile seizures.

Flu illness can cause high fever and febrile seizures in children. Flu vaccine can protect children against flu illness and its complications (2017 Recommended Immunizations for Children from Birth Through 6 Years Old [269 KB, 2 pages]).  Flu vaccine cannot cause the flu because of the way that it’s made. See Misconceptions about Seasonal Flu and Flu Vaccines for more information.

Several studies of children in the United States have been conducted to see if there is an increased risk for febrile seizures following flu vaccination.

The CDC carefully reviewed the data on febrile seizures and considered the benefits of vaccinating children against these illnesses, and decided that no changes in the childhood immunization recommendations should be made.

CDC and the Food and Drug Administration (FDA) closely monitor the safety of vaccines approved for use in the United States. CDC uses two primary systems to monitor the safety of flu vaccines:

  1. Vaccine Adverse Event Reporting System (VAERS): an early warning system that helps CDC and FDA monitor problems following vaccination. Anyone can report possible vaccine side effects to VAERS. Generally, VAERS reports cannot determine if an adverse event was caused by a vaccine, but these reports can help determine if further investigations are needed.
  2. Vaccine Safety Datalink (VSD): A collaboration between CDC and eight integrated health care organizations which conducts ongoing vaccine safety monitoring and in depth analysis of vaccine safety data.

Learn more about vaccine safety monitoring at Vaccine Safety Monitoring at CDC.

Yes. Febrile seizures following vaccination can be reported to CDC via the Vaccine Adverse Event Reporting System (VAERS). Your doctor might file this report, or you can do it yourself through the VAERS website.

To learn more about febrile seizures, visit the following sites:

Guillain-Barré syndrome (GBS) is a rare disorder in which a person’s own immune system damages their nerve cells, causing muscle weakness and sometimes paralysis. GBS can cause symptoms that usually last for a few weeks. Most people recover fully from GBS, but some people have long-term nerve damage. In very rare cases, people have died of GBS, usually from difficulty breathing. In the United States, an estimated 3,000 to 6,000 people develop GBS each year.

The exact cause of GBS is unknown, but about two-thirds of people who develop GBS experience symptoms several days or weeks after they have been sick with diarrhea or a respiratory illness. Infection with the bacterium Campylobacter jejuni is one of the most common risk factors for GBS. People also can develop GBS after having the flu or other infections (such as cytomegalovirus and Epstein Barr virus). On very rare occasions, they may develop GBS in the days or weeks after getting a vaccination.

Anyone can develop GBS; however, it is more common among older adults. The incidence of GBS increases with age, and people older than 50 years are at greatest risk for developing GBS.

The background rate for GBS in the Unites States is about 80 to 160 cases of GBS each week, regardless of vaccination. The data on the association between GBS and seasonal flu vaccination are variable and inconsistent across flu seasons. If there is an increased risk of GBS following flu vaccination it is small, on the order of one to two additional GBS cases per million doses of flu vaccine administered.

CDC and the Food and Drug Administration (FDA) closely monitor the safety of vaccines approved for use in the United States. CDC uses two primary systems to monitor the safety of flu vaccines:

  1. Vaccine Adverse Event Reporting System (VAERS): an early warning system that helps CDC and FDA monitor problems following vaccination. Anyone can report possible vaccine side effects to VAERS. Reports are welcome from all concerned individuals: for example, patients, parents, health care providers, pharmacists and vaccine manufacturers. Generally, VAERS reports cannot determine if an adverse event was caused by a vaccine, but these reports can help determine if further investigations are needed.
  1. Vaccine Safety Datalink (VSD): A collaboration between CDC and eight integrated health care organizations which conducts ongoing vaccine safety monitoring and research.

Learn more about vaccine safety monitoring at Vaccine Safety Monitoring at CDC.

Vaccine Effectiveness

Influenza (flu) vaccine effectiveness (VE) can vary. The protection provided by a flu vaccine varies from season to season and depends in part on the age and health status of the person getting the vaccine and the similarity or “match” between the viruses in the vaccine and those in circulation. During years when the flu vaccine match is good, the benefits of flu vaccination will vary, depending on factors like the characteristics of the person being vaccinated (for example, their health and age), what influenza viruses are circulating that season and, potentially, which type of flu vaccine was used. For more information, see Vaccine Effectiveness – How well does the Flu Vaccine Work. For information specific to this season, visit About the Current Flu Season.

Some people who get vaccinated may still get sick. However, flu vaccination has been shown in some studies to reduce severity of illness in people who get vaccinated but still get sick. A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized adults with flu. Another study in 2018 showed that a vaccinated adult who was hospitalized with flu was 59 percent less likely to be admitted to the ICU than someone who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.

No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. That’s why it’s best to get vaccinated before influenza viruses start to spread in your community.

Some people who get vaccinated may still get sick. However, flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick:

  • A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized flu patients.
  • Another study in 2018 showed that a vaccinated adult who was hospitalized with flu was 59% less likely to be admitted to an intensive care unit (ICU) than someone who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.

In addition, it’s important to remember that flu vaccine protects against three or four different viruses and multiple viruses usually circulate during any one season. For these reasons, CDC continues to recommend flu vaccination for everyone 6 months and older even if vaccine effectiveness against one or more viruses is reduced.

CDC conducts studies each year to determine how well the influenza (flu) vaccine protects against flu illness. While vaccine effectiveness (VE) can vary, recent studies show that flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well-matched to the flu vaccine. In general, current flu vaccines tend to work better against influenza B and influenza A(H1N1) viruses and offer lower protection against influenza A(H3N2) viruses. See “Does flu vaccine effectiveness vary by type or subtype?” and “Why is flu vaccine typically less effective against influenza A H3N2 viruses?” for more information.

How well the flu vaccine works (or its ability to prevent flu illness) can vary from season to season. The vaccine’s effectiveness also can vary depending on who is being vaccinated. At least two factors play an important role in determining the likelihood that flu vaccine will protect a person from flu illness: 1) characteristics of the person being vaccinated (such as their age and health), and 2) the similarity or “match” between the flu viruses the flu vaccine is designed to protect against and the flu viruses spreading in the community. During years when the flu vaccine is not well matched to circulating influenza viruses, it is possible that little or no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, it is possible to measure substantial benefits from flu vaccination in terms of preventing flu illness and complications. However, even during years when the flu vaccine match is good, the benefits of flu vaccination will vary, depending on various factors like the characteristics of the person being vaccinated, what influenza viruses are circulating that season and even, potentially, which type of flu vaccine was used.

Each flu season researchers try to determine how well flu vaccines work as a public health intervention. Estimates of how well a flu vaccine works can vary based on study design, outcome(s) measured, population studied and the season in which the flu vaccine was studied. These differences can make it difficult to compare one study’s results with another’s.

While determining how well a flu vaccine works is challenging, in general, recent studies have supported the conclusion that flu vaccination benefits public health, especially when the flu vaccine is well matched to circulating flu viruses. CDC uses mathematical modeling to estimate the burden of illness associated with influenza in terms of cases, hospitalizations and deaths.  These figures are then used to assess the burden of illness averted by flu vaccines, i.e., how many influenza cases, hospitalizations and deaths are averted in the United States each year as a result of flu vaccination. An explanation of U.S. flu burden and burden averted by seasonal flu vaccines is available from CDC’s Flu Burden page.

Seasonal flu vaccines are designed to protect against infection and illness caused by the three or four influenza viruses (depending on vaccine) that research indicates will be most common during the flu season. “Trivalent” flu vaccines are formulated to protect against three flu viruses, and “quadrivalent” flu vaccines protect against four flu viruses. Flu vaccines do NOT protect against infection and illness caused by other viruses that also can cause flu-like symptoms. There are many other viruses besides flu viruses that can result in flu-like illness* (also known as influenza-like illness or “ILI”) that spread during the flu season. These non-flu viruses include rhinovirus (one cause of the “common cold”) and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children, and a major cause of severe respiratory illness in adults aged 65 years and older.

Yes.  The amount of protection provided by flu vaccines may vary by influenza virus type or subtype even when recommended flu vaccine viruses and circulating influenza viruses are alike (well matched). Since 2009, VE studies looking at how well the flu vaccine protects against medically attended illness have suggested that when vaccine viruses and circulating flu viruses are well-matched, flu vaccines provide better protection against influenza B or influenza A (H1N1) viruses than against influenza A (H3N2) viruses. A study [505 KB, 10 pages] that looked at a number of VE estimates from 2004-2015 found average VE of 33% (CI = 26%–39%) against illnesses caused by H3N2 viruses, compared with 61% (CI = 57%–65%) against H1N1 and 54% (CI = 46%–61%) against influenza B virus illnesses. VE estimates were lower when vaccine viruses and circulating viruses were different (not well-matched). The same study found pooled VE of 23% (95% CI: 2% to 40%) against H3N2 viruses when circulating influenza viruses were significantly different from (not well-matched to) the recommended influenza A(H3N2) vaccine component.

There are a number of reasons why flu vaccine effectiveness against influenza A(H3N2) viruses may be lower.

  1. While all influenza viruses undergo frequent genetic changes, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses (i.e., antigenic change) compared with influenza A(H1N1) and influenza B viruses. That means that between the time when the composition of the flu vaccine is recommended and the flu vaccine is delivered, H3N2 viruses are more likely than H1N1 or influenza B viruses to have changed in ways that could impact how well the flu vaccine works.
  2. Growth in eggs is part of the production process for most seasonal flu vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses tend to be more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called “egg-adapted changes” are present in vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Other vaccine production technologies, e.g., cell-based vaccine production or recombinant flu vaccines, circumvent this shortcoming associated with the use of egg-based candidate vaccine viruses in egg-based production technology, but CDC also is using advanced molecular techniques to try to get around this short-coming.

In numerous studies since 2010, flu vaccines have helped protect adults 65 years of age and older against influenza A(H1N1) viruses and both lineages of influenza B viruses. Specifically, flu vaccines have reduced the risk of medically attended illness caused by H1N1 or B by more than 60% on average among people age65 and older (1). Flu vaccines also have reduced the risk of flu hospitalization among adults age 65 and older by 54% against A(H1N1) viruses and by 31% against influenza B viruses on average (2).

However, protection against influenza A(H3N2) flu viruses has been less consistent. On average, flu vaccines have reduced the risk of doctor visits with A(H3N2) flu by 24% and reduced the risk of hospitalization with A(H3N2) flu by 33% in adults age 65 and older (1,2). During seasons when the H3N2 vaccine component has been like (well-matched to) the flu viruses circulating in the community, the benefit from flu vaccination has been higher. During these seasons, flu vaccine reduced the risk of hospitalizations with A(H3N2) flu by 43% on average (2). But when the vaccine component was less similar to viruses in the community, the protection has dropped to 14% (2).

  1. Edward A. Belongia, Danuta M. Skowronski, Huong Q. McLean et al. Repeated annual influenza vaccination and vaccine effectiveness: review of evidence. Expert Review of Vaccines. 2017 Jun; 16(7): 723-36. doi: 10.1080/14760584.2017.1334554.
  2. Marc Rondy, Nathalie El Omeiri, Mark G. Thompson, et al. Effectiveness of influenza vaccines in preventing severe influenza illness among adults: A systemic review and meta-analysis of test-negative design case-control studies. Journal of Infection. Sept 2017; 65: 381-394. doi: 10.1016/j.jinf.2017.09.010

Despite the fact that some older adults (65 years of age and older) have weaker immune responses to the influenza A (H3N2) component of flu vaccines, there are many reasons why people in that age group should be vaccinated each year.

  • First, people aged 65 and older are at increased risk of serious illness, hospitalization and death from the flu.
  • Second, while the effectiveness of flu vaccines can be lower among some older people (particularly against influenza A(H3N2) viruses), there are seasons when significant benefit can be observed (this is particularly true against influenza A(H1N1) and influenza B viruses).
  • Third, flu vaccine may protect against more serious outcomes like hospitalization and death. For example, one study concluded that one death was prevented for every 4,000 people vaccinated against flu (1).
  • Flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick.
  • In frail elderly adults, hospitalizations can mark the beginning of a significant decline in overall health and mobility, potentially resulting in loss of the ability to live independently or to complete basic activities of daily living. While the protection elderly adults obtain from flu vaccination can vary significantly, a yearly flu vaccination is still the best protection currently available against flu.
  • Fourth, it’s important to remember that people who are 65 and older are a diverse group and often are different from one another in terms of their overall health, level of activity and mobility, and behavior when it comes to seeking medical care. This group includes people who are healthy and active and have responsive immune systems, as well as those who have underlying medical conditions that may weaken their immune system and their bodies’ ability to respond to vaccination. Therefore, when evaluating the benefits of flu vaccination, it is important to look at a broader picture than what one study’s findings can present.
  1. Fireman B, Lee J, Lewis N et al. Influenza vaccination and mortality: differentiating vaccine effects from bias. Am J Epidemiol. 2019 Sep; 170(5): 650-6. doi: 10.1093/aje/kwp173.

Vaccination has been found in most seasons to provide a similar level of protection against flu illness in children to that seen among healthy adults.

In several studies, flu vaccine effectiveness was higher among children who received two doses of flu vaccine the first season that they were vaccinated (as recommended) compared to “partially vaccinated” children who only received a single dose of flu vaccine. However, in some seasons, partially vaccinated children still receive some protection.

In addition to preventing illness, flu vaccine can prevent severe, life-threatening complications in children, for example:

  • A 2014 study showed that flu vaccine reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74% during flu seasons from 2010-2012.
  • In 2017, a study in the journal Pediatrics was the first of its kind to show that flu vaccination also significantly reduced a child’s risk of dying from the flu. The study, which looked at data from four flu seasons between 2010 and 2014, found that flu vaccination reduced the risk of flu-associated death by half (51 percent) among children with underlying high-risk medical conditions and by nearly two-thirds (65 percent) among healthy children.

CDC typically presents vaccine effectiveness (VE) as a single point estimate: for example, 60%. This point estimate represents the reduction in risk provided by the flu vaccine. CDC vaccine effectiveness studies measure two outcomes: laboratory confirmed flu illness that results in a doctor’s visit or laboratory-confirmed flu that results in hospitalization. For these outcomes, a VE point estimate of 60% means that on average the flu vaccine reduces a person’s risk of a flu outcome by 60%.

In addition to the VE point estimate, CDC also provides a “confidence interval” (CI) for this point estimate, for example, 60% (95% CI: 50%-70%). The confidence interval provides a lower boundary for the VE estimate (e.g., 50%) as well as an upper boundary (e.g., 70%). One way to interpret a 95% confidence interval is that if CDC were to repeat this study 100 times and calculate 100 confidence intervals, 95 times out of 100, the confidence interval would contain the true VE value. A simpler interpretation is that there is a 95% chance that the true VE lies within the confidence interval – therefore, there is still the possibility that five times out of 100 (a 5% chance) the true VE value could fall outside of the 50%-70% confidence interval.

Confidence intervals are important because they provide context for understanding the precision or exactness of a VE point estimate. The wider the confidence interval, the less exact the point value estimate of vaccine effectiveness becomes. Take, for example, a VE point estimate of 60%. If the confidence interval of this point estimate is 50%-70%, then we can have greater certainty that the true protective effect of the flu vaccine is near 60% than if the confidence interval was between 10% and 90%. Furthermore, if a confidence interval crosses zero, for example, (-20% to 60%), then the point value estimate of VE provided is “not statistically significant.” People should be cautious when interpreting VE estimates that are not statistically significant because such results cannot rule out the possibility of zero VE (i.e., no protective benefit). The width of a confidence interval is related in part to the number of participants in the study, and so studies that provide more precise estimates of VE (and consequently, have a narrower confidence interval) typically include a large number of participants.

A recent report examining studies from 2010-11 to 2014-15 concluded that the effectiveness of a flu vaccine may be influenced by vaccination the prior season or during many prior seasons (1). In some seasons, protection against influenza A(H3N2) virus illness may have been lower for people vaccinated in the current season and the prior season compared to those who had only been vaccinated in the current season. This fits with findings on immune response to vaccination that suggest repeated influenza vaccination can weaken immune response to vaccination and especially to the H3N2 vaccine component. However, repeated annual vaccination also can be beneficial during some seasons, since sometimes people retain and carry over immune protection from one season to the next. During some seasons, people who missed getting vaccinated still had residual protection against influenza illness.

Information regarding flu vaccination history is particularly important to these types of evaluations, and can be difficult to confirm, as accurate vaccination records are not always readily available. People who choose to get vaccinated every year may have different characteristics and susceptibility to flu compared to those who do not seek vaccination every year. CDC thinks that these findings merit further investigation to understand the immune response to repeat vaccination. CDC supports continued efforts to monitor the effects of repeat vaccination each year. However, based on the substantial burden of flu in the United States, and on the fact that most studies point to vaccination benefits, CDC recommends that yearly flu vaccination remains the first and most important step in protecting against flu and its complications.

  1. Edward A. Belongia, Danuta M. Skowronski, Huong Q. McLean et al. Repeated annual influenza vaccination and vaccine effectiveness: review of evidence. Expert Review of Vaccines. 2017 Jun; 16(7): 723-36. doi: 10.1080/14760584.2017.1334554.

Vaccine effectiveness studies that measure different outcomes are conducted to better understand the different kinds of benefits provided by vaccination. Ideally, public health researchers want to know how well flu vaccines work to prevent illness resulting in a doctor visit, or illness resulting in hospitalization, and even death associated with the flu, to evaluate the benefits of vaccination against illness of varying severity.  Because estimates of vaccine effectiveness may vary based on the outcome measured (in addition to season, population studied and other factors), results should be compared between studies that used the same outcome for estimating vaccine effectiveness.

Recent studies show flu vaccine can reduce the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well matched to the flu vaccine viruses.

Scientists continue to work on better ways to design, conduct and evaluate non-randomized (i.e., observational) studies to assess how well flu vaccines work. CDC has been working with researchers at universities and hospitals since the 2003-2004 flu season to estimate how well flu vaccine works through observational studies using laboratory-confirmed flu as the outcome. These studies currently use a very accurate and sensitive laboratory test known as real-time RT-PCR (reverse transcription polymerase chain reaction) to confirm medically-attended flu virus infections as a specific outcome. CDC’s studies are conducted in five sites across the United States to gather more representative data. To assess how well the vaccine works across different age groups, CDC’s studies of flu vaccine effects have included all people aged 6 months and older recommended for an annual flu vaccination. Similar studies are being done in Australia, Canada and Europe. More recently, CDC set up a second network, called the Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN), that looks at how well flu vaccine protects against flu-related hospitalization among adults aged 18 and older.

The large numbers of flu-associated illnesses and deaths in the United States, combined with the evidence from many studies that show flu vaccines help to protect against flu illness and its potentially serious complications, support the current U.S. flu vaccination recommendations.

Getting a flu vaccine each year is the best way to prevent the flu. In addition to getting the flu shot, people should take the same everyday preventive actions to prevent the spread of flu, including covering coughs, washing hands often, and avoiding people who are sick. Antiviral drugs are an important second line of defense to treat the flu. These drugs are not a substitute for vaccination and must be prescribed by a health care provider.

More information on Vaccine Selection.

Different Flu Vaccine (Shot) Types

For the 2020-2021 flu season, the Advisory Committee on Immunization Practices (ACIP) recommends annual influenza (flu) vaccination for everyone 6 months and older with any licensed, influenza vaccine that is appropriate for the recipient’s age and health status, including inactivated influenza vaccine (IIV), recombinant influenza vaccine (RIV), or live attenuated nasal spray influenza vaccine (LAIV4) with no preference expressed for any one vaccine over another.

There are many vaccine options to choose from, but the most important thing is for all people 6 months and older to get a flu vaccine every year. If you have questions about which vaccine is best for you, talk to your doctor or other health care professional.

For people younger than 65 years, CDC does not preferentially recommend any licensed, age-appropriate influenza (flu) vaccine over another during the 2022-2023 flu season. Options for this age group include inactivated influenza vaccine [IIV], recombinant influenza vaccine [RIV], or live attenuated influenza vaccine (LAIV), with no preference for any flu vaccine over another.

New for this season: For people 65 years and older, there are three flu vaccines that are preferentially recommended over standard-dose, unadjuvanted flu vaccines. These are Fluzone High-Dose Quadrivalent vaccineFlublok Quadrivalent recombinant flu vaccine and Fluad Quadrivalent adjuvanted flu vaccine. More information is available at Flu & People 65 Years and Older.

All flu vaccines for the 2022-2023 season are quadrivalent vaccines, designed to protect against four different flu viruses, including two influenza A viruses and two influenza B viruses. Different vaccines are licensed for use in different age groups, and some vaccines are not recommended for some groups of people.

Available flu vaccines include:

  • Standard-dose flu shots that are manufactured using virus grown in eggs. Several different brands of standard dose flu shots are available, including Afluria Quadrivalent, Fluarix Quadrivalent, FluLaval Quadrivalent, and Fluzone Quadrivalent. These vaccines are approved for use in children as young as 6 months. Most flu shots are given in the arm (muscle) with a needle. Afluria Quadrivalent can be given either with a needle (for people 6 months and older) or with a jet injector (for people 18 through 64 years only).
  • cell-based flu shot (Flucelvax Quadrivalent) containing virus grown in cell culture, which is approved for people 6 months and older. This vaccine is completely egg-free.
  • recombinant flu shot (Flublok Quadrivalent) which is a completely egg-free flu shot that is made using recombinant technology and is approved for use in people 18 years and older. This shot is made without flu viruses and contains three times the antigen (the part of the vaccine that helps your body build up protection against flu viruses) than other standard-dose inactivated flu vaccines, to help create a stronger immune response.
  • An egg-based high dose flu shot (Fluzone High-Dose Quadrivalent), which is approved for use in people 65 years and older. This vaccine contains four times the antigen (the part of the vaccine that helps your body build up protection against flu viruses) than other standard-dose inactivated flu vaccines, to help create a stronger immune response.
  • An egg-based adjuvanted flu shot (Fluad Quadrivalent), which is approved for people 65 years and older. This vaccine is made with an adjuvant (an ingredient that helps create a stronger immune response).
  • An egg-based live attenuated flu nasal spray vaccine (FluMist Quadrivalent) made with attenuated (weakened) live flu viruses, which is approved for use in people 2 years through 49 years. This vaccine is not recommended for use in pregnant people, immunocompromised people, or people with certain medical conditions.

All influenza vaccines contain antigen derived from the same influenza viruses, with the one difference being that trivalent vaccines have 3 different antigens and quadrivalent vaccines have four different antigens (the same three that are in the trivalent vaccines, plus one more). However, aside from the antigen composition, different influenza vaccines are manufactured differently and different preparations have different indications as licensed by the FDA. In particular, each is licensed for a specific age range. All recipients should receive a vaccine that is appropriate for their age. In addition, LAIV (the nasal spray flu vaccine) is not recommended for use in some populations.

All nasal spray influenza vaccines for the 2019-2020 season will contain four influenza viruses: an influenza A (H1N1) virus, an influenza A (H3N2) virus and two influenza B viruses.

Learn more about the vaccine composition for the 2019-20 influenza season.

The nasal spray is approved for use in non-pregnant individuals, 2 years through 49 years of age. People with certain medical conditions should not receive the nasal spray influenza vaccine.

Some people should not get the nasal spray influenza vaccine:

  • Children younger than 2 years
  • Adults 50 years and older
  • Pregnant women
  • People with a history of severe allergic reaction to any component of the vaccine or to a previous dose of any influenza vaccine
  • Children 2 years through 17 years of age who are receiving aspirin- or salicylate-containing medications.
  • People with weakened immune systems (immunosuppression)
  • Children 2 years through 4 years who have asthma or who have had a history of wheezing in the past 12 months.
  • People who have taken influenza antiviral drugs within the previous 48 hours.
  • People who care for severely immunocompromised persons who require a protected environment (or otherwise avoid contact with those persons for 7 days after getting the nasal spray vaccine).

In addition, the following conditions are precautions to the use of the nasal spray influenza vaccine:

  • Asthma in people aged 5 years and older.
  • Other underlying medical conditions that can put people at higher risk of serious influenza complications. These include conditions such as lung disease, heart disease (except isolated hypertension), kidney disease (like diabetes), kidney or liver disorders, neurologic/neuromuscular, or metabolic disorders. “People at High Risk of Developing Flu–Related Complications.”
  • Moderate or severe acute illness with or without fever.
  • Guillain-Barré Syndrome within 6 weeks following a previous dose of influenza vaccine.

Influenza vaccine effectiveness (VE) can vary from year to year, among different age and risk groups, by vaccine type, and even by virus type and subtype. Prior to the 2009 influenza pandemic, the nasal spray vaccine was found to be effective against different influenza viruses.  After the 2009 pandemic, several U.S. studies among 2 through 17-year-olds found that the nasal spray vaccine was effective against influenza B viruses, and was similarly effective against H3N2 viruses as inactivated influenza vaccines, but was less effective than inactivated influenza vaccines against the 2009 pandemic H1N1 viruses, leading ACIP and CDC to recommend against use of the nasal spray vaccine in 2016.

Since the 2017-2018 season, the manufacturer of nasal spray vaccine has used new H1N1 vaccine viruses in production. Some data suggest this will result in improved effectiveness of against H1N1. However, no U.S. studies have evaluated effectiveness of this updated vaccine component against H1N1 viruses. ACIP and CDC voted to resume the recommendation for the use of nasal spray vaccines based on evidence suggesting that the new H1N1 component will result in improved effectiveness of the vaccine against these viruses. There is no expressed preference for any influenza shot or the nasal spray vaccine.

The nasal spray vaccine is an important option for providers, patients and parents in the U.S. and other countries where it continues to be recommended. How Well Does the Seasonal Flu Vaccine Work?

LAIV is not recommended for people with some kinds of chronic health conditions because the safety and effectiveness of this vaccine in people with those conditions has not been established.

Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP) – United States, 2015-2016 Influenza Season – August 7, 2015 includes a list of contraindications and precautions for the nasal spray vaccine.


Pregnant and postpartum women do not need to avoid contact with persons recently vaccinated with the nasal spray influenza vaccine. However, the nasal spray influenza vaccine should not be given to women who are pregnant. Postpartum women can receive an influenza shot or the nasal spray influenza vaccine.

Breastfeeding is not a contraindication for the nasal spray vaccine. Breastfeeding mothers younger than 50 years can get the nasal spray influenza vaccine as long as they do not have any contraindication to getting that vaccine. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP) – United States, 2015-2016 Influenza Season – August 7, 2015 includes a list of contraindications and precautions for the nasal spray vaccine.

The nasal spray influenza vaccine can be given to people with mild illnesses (e.g., diarrhea or mild upper respiratory tract infection with or without fever). However, nasal congestion might limit delivery of the vaccine to the nasal lining. Delaying vaccination with LAIV until the nasal congestion is reduced or use of an age-appropriate injectable vaccine instead of LAIV should be considered. People with moderate or severe illness, with or without fever, should generally wait to be vaccinated until they have recovered. Your health care provider can provide advice about when to get vaccinated if you are feeling ill.

Influenza vaccines do not cause influenza illness. The nasal spray influenza vaccine does contain live viruses. However, the viruses are attenuated (weakened), so that they will not cause influenza illness. The weakened viruses are also cold-adapted, which means they are designed to only multiply at the cooler temperatures found within the nose. The viruses cannot infect the lungs or other areas where warmer temperatures exist.

While an influenza vaccine cannot give you influenza illness, there are different side effects that may be associated with getting an influenza shot or a nasal spray influenza vaccine.

These side effects are mild and short-lasting, especially when compared to symptoms of bad case of influenza.

The viruses in the nasal spray vaccine are weakened and do not cause severe symptoms often associated with influenza illness. In children, side effects from the nasal spray may include:

  • Runny nose
  • Wheezing
  • Headache
  • Vomiting
  • Muscle aches
  • Fever (low grade)

In adults, side effects from the nasal spray vaccine may include:

  • Runny nose
  • Headache
  • Sore throat
  • Cough

If these problems occur, they begin soon after vaccination and usually are mild and short-lived. People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears. Rarely, people can experience a severe allergic reaction after an influenza vaccine (or any vaccine); there are about 1-2 cases of severe allergic reactions per million influenza vaccine doses administered and these reactions can be treated with medication. People who think that they have been injured by an influenza vaccine can file a claim with the National Vaccine Injury Compensation Program (VICP).

More information about the safety of influenza vaccines is available at Influenza Vaccine Safety.

Note: There is no recommendation for pregnant women or people with pre-existing medical conditions to get special permission or written consent from their doctor or health care professional for influenza vaccination if they get vaccinated at a worksite clinic, pharmacy or other location outside of their physician’s office. For more information, visit Misconceptions about Seasonal Flu and Flu Vaccines.

Fluzone High-Dose Quadrivalent is the only licensed high-dose inactivated influenza (flu) vaccine; it is manufactured by Sanofi Pasteur Inc. Fluzone High-Dose Quadrivalent is a four-ingredient (quadrivalent) flu vaccine approved for people 65 years and older.

In the United States, Fluzone High-Dose Quadrivalent is licensed only for persons aged 65 years and older. Fluzone High-Dose Quadrivalent is not recommended for persons with a history of severe allergic reaction to the vaccine or to ingredients other than eggs. Information about vaccine ingredients is located in package inserts from each manufacturer.

Fluzone High-Dose Quadrivalent contains four times the antigen, the part of the vaccine that helps your body build up protection against flu viruses, then Fluzone Quadrivalent and other standard-dose inactivated flu vaccines. Both are quadrivalent vaccines. The higher dose of antigen in the vaccine is intended to give older people a better immune response, and therefore, better protection against flu. Both these products are flu vaccines produced by the same manufacturer. There are a number of other flu vaccines produced by other manufacturers.

Data from clinical trials comparing Fluzone (a trivalent standard dose vaccine) to Fluzone High-Dose (a trivalent high-dose vaccine) among persons aged 65 years and older indicate that a stronger immune response (i.e., higher antibody levels) occurs after vaccination with Fluzone High-Dose. A study published in the New England Journal of Medicine indicated that the high-dose vaccine was 24% more effective in preventing flu in adults 65 years and older relative to a standard-dose vaccine. Another study published in The Lancet Respiratory Medicine reported that Fluzone High-dose was associated with a lower risk of hospital admissions compared with standard-dose Fluzone in adults 65 years and older, especially those living in long-term care facilities. This study was conducted during the 2013-14 flu season among more than 38,000 resident of 823 nursing homes in 38 states.

For the 2020-21 season, trivalent Fluzone High-Dose will be replaced by Fluzone High-Dose Quadrivalent. Data comparing the effectiveness of Fluzone High-Dose Quadrivalent with standard-dose inactivated vaccines are not yet available.

A study published in the Journal of Infectious Diseases among Medicare beneficiaries aged 65 years and older found that Fluzone High-Dose or Flucelvax provided greater protection against flu-related hospitalizations than standard-dose, egg based vaccine and FLUAD.

There are many vaccine options to choose from, but the most important thing is for all people 6 months and older to get a flu vaccine every year. If you have questions about which vaccine is best for you, talk to your doctor or other health care professional.

Some adverse events were reported more frequently after vaccination with trivalent Fluzone High-Dose than after standard-dose inactivated flu vaccines. The most common adverse events experienced during clinical studies were mild and temporary, and included pain, redness at the injection site, headache, muscle aches, and malaise. Most people had minimal or no adverse events after receiving the Fluzone High-Dose. In a study comparing Fluzone High-Dose Quadrivalent with trivalent Fluzone High-Dose, some of these side effects were slightly more common with the quadrivalent vaccine, but most were mild and resolved within a few days.

A quadrivalent influenza vaccine is designed to protect against four different influenza viruses; two influenza A viruses and two influenza B viruses.

Learn more about the vaccine composition for the influenza season.

For many years, influenza vaccines were designed to protect against three different influenza viruses (trivalent vaccines). Trivalent vaccines include an influenza A (H1N1) virus, an influenza A (H3N2) virus and one influenza B virus. Experts had to choose one B virus, even though there are two different lineages of B viruses that both circulate during most seasons. Adding another B virus to the vaccine aims to give broader protection against circulating influenza viruses.

Different vaccines are approved for different age groups. There is a quadrivalent influenza shot that can be given to children as young as 6 months of age. Other quadrivalent influenza shots are approved for people 3 years and older. More information on approved influenza vaccines for the 2019-2020 influenza season, and age indications for each vaccine are available in CDC’s Table: U.S. Influenza Vaccine Products for the 2019-20 Season.

The quadrivalent nasal spray vaccine is approved for use in non-pregnant individuals, 2 years through 49 years of age. People with certain medical conditions should not receive the nasal spray influenza vaccine.

Different influenza vaccines are approved for use in different age groups. In addition, some vaccines are not recommended for certain groups of people. Factors that can determine a person’s suitability for vaccination, or vaccination with a particular vaccine, include a person’s age, health (current and past) and any allergies to influenza vaccine or its components.

Most influenza vaccine in the United States is now quadrivalent. Of the 162 to 169 million doses of influenza vaccine projected to be available for the 2019-2020 season, manufacturers estimate that 129 million to 138 million doses will be quadrivalent influenza vaccine.

Yes. Influenza vaccines that protect against four influenza viruses have a safety profile similar to seasonal influenza vaccines made to protect against three viruses, with similar—mostly mild—side effects. Like all seasonal influenza vaccines, vaccines that protect against four influenza viruses are monitored annually for their safety and effectiveness.

Quadrivalent vaccine cannot cause influenza illness because they contain ‘inactivated’ (killed) virus, attenuated (weakened) virus, or influenza virus components that are made using recombinant methods rather than viruses.

Don’t delay getting vaccinated if you cannot locate a quadrivalent vaccine. The important thing is to get vaccinated against influenza. Find flu vaccine near you: Vaccine Finder. Call ahead to ask about availability if you are interested in a specific type of vaccine.

FLUAD is a standard-dose, three-component (trivalent) inactivated flu vaccine, manufactured by Seqirus that contains an adjuvant. FLUAD is designed specifically for people 65 years and older. It is manufactured using an egg-based process (like most flu vaccines), and is formulated with the adjuvant MF59. An adjuvant is an ingredient added to a vaccine that helps create a stronger immune response to vaccination.

MF59 is an oil-in-water emulsion of squalene oil. Squalene, a naturally occurring substance found in humans, animals and plants, is highly purified for the vaccine manufacturing process. FLUAD is approved for use among people 65 years and older, who often have a lower protective immune response after flu vaccination compared to younger, healthier people.

An adjuvant is an ingredient of a vaccine that helps promote a better immune response. Adjuvants also can reduce the amount of virus needed for production of a vaccine, which can allow for greater supplies of vaccine to be manufactured.

FLUAD was initially approved in Italy in 1997, and at the time of its U.S. approval in November 2015, had been licensed in 38 countries, including Canada and 15 European countries.

In the United States, FLUAD is licensed only for persons aged 65 years and older. FLUAD is not recommended for persons with a history of severe allergic reaction to the vaccine or to components other than eggs. Information about vaccine components is located in package inserts from each manufacturer.

There are many benefits of flu vaccine with adjuvant (FLUAD) compared to unadjuvanted seasonal flu vaccines for adults 65 years and older. Studies that have tested FLUAD’s ability to generate an immune response against an influenza virus (immunogenicity) have found that antibody levels were comparable to levels induced by unadjuvanted trivalent seasonal flu vaccines (e.g., Agriflu). However, an observational study conducted in Canada among adults 65 years of age and older during the 2011-2012 flu season found that FLUAD was significantly more effective in preventing laboratory-confirmed influenza compared with an unadjuvanted standard-dose inactivated influenza vaccine.

To date, there have been no randomized studies comparing FLUAD with High-Dose flu vaccine (brand name, Fluzone).

There are many vaccine options to choose from, but the most important thing is for all people 6 months and older to get a flu vaccine every year. If you have questions about which vaccine is best for you, talk to your doctor or other health care professional.

The CDC and its Advisory Committee on Immunization Practices have not expressed a preference for any flu vaccine indicated for people 65 and older. CDC recommends flu vaccination as the first and most important step in protecting against the flu.

Some adverse events (which are also reported after regular flu vaccines) were reported more frequently after vaccination with FLUAD. The most common adverse events experienced during clinical studies were mild to moderate and were temporary, and included pain, redness at the injection site, headache, muscle aches, and malaise.

More information about FLUAD is available on the Food and Drug Administration (FDA) web site.

In addition to FLUAD, one other influenza vaccine is licensed specifically for people 65 years and older. The high dose vaccine contains four times the amount of antigen as the standard-dose inactivated influenza vaccine, and has been associated with a stronger immune response following vaccination and better effectiveness than the regular dose flu vaccine in older people in a two-season randomized trial. People in this age group may also receive standard-dose, unadjuvanted influenza vaccines or the recombinant influenza vaccine. There is no preferential recommendation made for any flu vaccine formulation for this age group.

CDC studies conducted during previous flu seasons estimate that that between about 70 percent and 85 percent of seasonal flu-related deaths have occurred in people 65 years and older and between 54 percent and 70 percent of seasonal flu-related hospitalizations have occurred among people in that age group. However, older adults with weaker immune systems also may have a lower protective immune response after flu vaccination compared to younger, healthier people. This can result in lower vaccine effectiveness (i.e., a measure of how well the flu vaccine protects against flu illness), in these people. Newer flu vaccines made specifically for people 65 years of age attempt to improve the immune response and protection provided by flu vaccination in this age group.

‘Cell-based’ refers to how the flu vaccine is made. Most inactivated influenza vaccines are produced by growing influenza viruses in eggs. The influenza viruses used in the cell-based vaccine are grown in cultured cells of mammalian origin instead of in hens’ eggs.

Flucelvax Quadrivalent is the only cell-based inactivated influenza vaccine that has been licensed by the FDA for use during the 2019-2020 flu season.

Flucelvax Quadrivalent is licensed for use in people 4 years and older.

A cell-based flu vaccine was developed as an alternative to the egg-based manufacturing process. Cell-culture technology is potentially more flexible than the traditional technology, which relies upon adequate supply of eggs. In addition, the cell-based flu vaccine that uses cell-based candidate vaccine viruses (CVVs) has the potential to offer better protection than traditional, egg-based flu vaccines as a result of being more similar to flu viruses in circulation.

While viruses used in Flucelvax Quadrivalent have been grown in cells since the vaccine first became available, prior to the 2019-2020 season some of the viruses provided to the manufacturer had been originally derived in eggs. For the 2019-2020 influenza season, all four flu viruses used in the Flucelvax Quadrivalent are cell-derived, making the vaccine egg-free.

In place of fertilized chicken eggs, the cell-based vaccine manufacturing process uses animal cells (Madin-Darby Canine Kidney, or MDCK cells) in liquid culture as a host for the growing influenza virus. For the 2019-20 season, the viruses provided to the manufacturer to be grown in cell culture are cell-derived rather than egg derived. Learn more about the cell-based flu vaccine manufacturing process: CDC’s How Flu Vaccines are Made.

Growing influenza viruses in eggs can introduce changes (called egg-adapted changes) that can cause differences between the viruses in the vaccine and the ones that are circulating. These changes can have important implications for the body’s immune response to vaccination. For example, these egg-adapted changes could cause the body’s immune system to produce antibodies that are less effective at preventing disease caused by the specific flu viruses in circulation. FDA approval of cell-grown CVVs will reduce egg-adapted changes and may result in vaccines containing virus that is more “like” wild-type circulating viruses. Therefore, the FDA’s approval of cell-based candidate vaccine viruses has the potential to improve the effectiveness of cell-based flu vaccines.

A potential advantage of cell culture technology is that it might permit faster start-up of the vaccine manufacturing process in the event of a pandemic. The cells used to manufacture Flucelvax Quadrivalent are kept frozen and “banked.” Cell banking assures an adequate supply of cells is readily available for vaccine production. Growing the influenza viruses in cell culture for the manufacture of Flucelvax Quadrivalent is not dependent on an egg supply. Also, as described above, cell-based flu vaccines that are produced using cell-based candidate vaccine viruses have the potential to improve the effectiveness of cell-based flu vaccines (compared to egg-based flu vaccines) by eliminating the kinds of egg adaptations that occur in egg-manufactured flu vaccines.

A clinical trial of the previous trivalent formulation of Flucelvax demonstrated effectiveness and safety among persons 18 through 49 years of age. In immunogenicity studies among people 18 years if age and older and 4 through 17 years of age, Flucelvax Quadrivalent was found to produce a similar immune response to the trivalent formulation. Post-vaccination symptoms were typical of those seen with other injectable influenza vaccines.

Cell culture technology has been used to produce other U.S.-licensed vaccines, including vaccines for rotavirus, polio, smallpox, hepatitis, rubella and chickenpox. Cell-based flu vaccines have been approved for use in multiple European countries.

Recombinant influenza vaccines are produced using recombinant technology. This method does not require an egg-grown vaccine virus and does not use chicken eggs in the production process. Currently, the recombinant influenza vaccine and the cell culture-based influenza vaccine are the only egg-free influenza vaccines licensed for use in the United States.

Flublok Quadrivalent is a quadrivalent recombinant influenza vaccine that was first licensed by the FDA in the United States for use in adults 18 years and older in 2017.  An earlier trivalent version was licensed in 2013, but has been replaced by the quadrivalent version.

Recombinant vaccines are produced differently from other vaccines.  Recombinant influenza vaccines do not use the influenza virus or chicken eggs in the manufacturing process. How Influenza (Flu) Vaccines Are Made is available for more information.

Flublok Quadrivalent is licensed for use in people 18 years of age and older.

People who are not within the FDA-approved age range (those younger than 18 years) and people with known severe allergic reactions to any component of the vaccine should not get Flublok Quadrivalent.

Because it is not dependent on an egg supply, the recombinant influenza vaccine manufacturing process might be faster than that of egg-based vaccines in the event of a pandemic or shortage of the eggs needed to grow influenza viruses, and is not limited by the selection of vaccine viruses that are adapted for growth in eggs.  This process also avoids mutations that can occur when viruses are grown in eggs, which can sometimes affect how well the finished vaccine works.  Learn more about the recombinant influenza vaccine manufacturing process on CDC’s How Flu Vaccines are Made.

In clinical studies, the safety of recombinant influenza vaccines was comparable to that of other injectable influenza vaccines.  The most common side effects reported after receipt of Flublok Quadrivalent were similar to those reported for other injectable vaccines and include pain and tenderness at the injection site, headache, fatigue, and muscle or joint aches.

For more information: the Flublok Quadrivalent package insert.

A jet injector is a medical device used for vaccination that uses a high-pressure, narrow stream of fluid to penetrate the skin instead of a hypodermic needle. Jet injectors may be powered by compressed gas or springs. The devices were invented in the 1960s and were used successfully in mass vaccination efforts to eradicate smallpox and other diseases. In the past, the devices were often used to vaccinate members of the armed forces.

One flu vaccine, AFLURIA Quadrivalent®, is approved for use with a jet injector this season. AFLURIA Quadrivalent® (i.e., four ingredient) flu vaccine provides protection against an influenza A(H1N1) virus, and influenza A(H3N2) virus and two influenza B viruses. For more information: What You Should Know for the 2020-2021 Season.

Flu vaccination with AFLURIA Quadrivalent® via jet injector is approved for use in people 18 through 64 years old. AFLURIA Quadrivalent may also be given by needle and syringe, for people 6 months old and older.

Clinical trial data used in FDA’s approval of the jet injector flu vaccine (i.e., the PharmaJet Stratis Needle-Free Injector used with the previous trivalent AFLURIA®) demonstrated that vaccination using this jet injector method provided a non-inferior (i.e., similar) level of immune protection compared to the same flu vaccine administered via traditional flu shot.

In adults 18 through 64 years old, the most common injection-site adverse reactions observed in a clinical study of the jet injector flu vaccine up to 7 days post-vaccination were tenderness, swelling, pain, redness, itching and bruising. The most common systemic adverse events within this period were myalgia (muscle pain), malaise (i.e., the feeling of discomfort or unease like when you are first getting sick), and headache.

Yes. The Afluria Quadrivalent® formulation used for administration by jet injector comes in multi-dose vials, which contain thimerosal. For more information, see Thimerosal Flu Vaccines.

Thimerosal is an ethyl mercury-based preservative used in vials that contain more than one dose of a vaccine (multi-dose vials) to prevent germs, bacteria and/or fungi from contaminating the vaccine.

Flu vaccines in multi-dose vials contain thimerosal to safeguard against contamination of the vial. Most single-dose vials and pre-filled syringes of flu shot and the nasal spray flu vaccine do not contain a preservative because they are intended to be used once.

A list of available flu vaccines and their thimerosal content is available.

Preservatives are used to protect vaccines packaged in multi-dose vials. Each time a vaccine dose is drawn from a multi-dose vial, bacteria or fungi can enter the vial. Receiving a vaccine contaminated with bacteria or fungi can be dangerous. Preservatives are needed to prevent contamination of multi-dose vials each time individual doses are drawn.

Thimerosal use in vaccines and other medical products has a record of being very safe. Data from many studies show no evidence of harm caused by the low doses of thimerosal in vaccines.

Please visit CDC’s webpage Thimerosal in Vaccines for more information on thimerosal, including Frequently Asked Questions about Thimerosal.

The influenza viruses in the seasonal flu vaccine are selected each year based on surveillance data indicating which viruses are circulating and forecasts about which viruses are the most likely to circulate during the coming season. The degree of similarity between available vaccine viruses and circulating viruses also is important. Vaccine viruses must be similar to the influenza viruses predicted to circulate most commonly during the upcoming season. Another important practical factor in the recommendation about what viruses to include in a flu vaccine is whether or not there is a good vaccine virus available; that is, a virus that could be used in vaccine production and which would likely protect against the viruses likely to circulate during the upcoming season. Historically, vaccine viruses were required by FDA to be isolated and grown in chicken eggs, but now the FDA allows vaccine viruses to be grown in cells, too. Regardless of how they are grown, vaccine viruses must be tested and available in time to allow for production of the large amount of vaccine virus needed to make vaccine. Occasionally, a suitable vaccine virus cannot be identified or developed in time to be included in the upcoming season’s vaccine.

There are a number of factors that can make getting a good vaccine virus for vaccine production challenging, including both scientific issues and issues of timing. FDA regulatory requirements now allow influenza vaccine viruses to be grown in chicken eggs or cells. Today, the majority influenza vaccines are still grown in chicken eggs. Unfortunately, some influenza viruses, like H3N2 viruses, grow poorly in eggs, making it difficult to obtain candidate vaccine viruses.

In terms of timing, in some years certain influenza viruses may not appear and spread until later in the influenza season, making it difficult to prepare a candidate vaccine virus in time for vaccine production. This can make vaccine virus selection very challenging.

As one of five WHO Collaborating Centers, CDC’s Influenza Division receives and tests thousands of influenza viruses from around the world each year and collaborates with other WHO Collaborating Centers and National Influenza Centers in the bi-annual seasonal vaccine virus selection process for the Southern and Northern Hemispheres. CDC plays a major role in testing and identifying new strains of influenza viruses through their global surveillance activities and then preparing candidate vaccine viruses. The Influenza Division provides this information to the directors of other WHO Collaborating Centers, essential regulatory laboratories and representatives of key national laboratories and academies and then participates in discussions regarding which viruses will be recommended for inclusion in flu vaccines. CDC also presents information to FDA’s advisory committee for their decision making and helps to identify vaccine viruses.

CDC’s Influenza Division collects and reports information on influenza activity in the United States each week. Laboratory studies of circulating influenza viruses allow CDC to evaluate how close a match there is between viruses in the vaccine and circulating viruses each season. Antigenic characterization is one process that can give an indication of the flu vaccine’s ability to produce an immune response against the influenza viruses circulating in people. Genetic characterization also can inform decision-making for vaccine virus selection. CDC also conducts studies to determine the effectiveness of the seasonal vaccine against circulating viruses. For more information, see Vaccine Effectiveness – How Well Does the Flu Vaccine Work? For more information about CDC’s surveillance and to access the weekly reports, visit Flu Activity and Surveillance.

As soon as a recommendation has been issued about what viruses should be included in the vaccine, private sector manufacturers begin the process of producing vaccine. In fact, some manufacturers may start growing one or more viruses for the vaccine even before a WHO or FDA decision is made based on what they think may be the recommended vaccine viruses. This allows manufacturers more time to make vaccine for the fall; the more time a manufacturer has to make vaccine, the greater the number of doses that can be produced.

It takes at least six months to produce large quantities of influenza vaccine. For vaccine to be delivered in time for vaccination to begin in the fall, manufacturers may begin to grow one or more of the vaccine viruses in January based on their best guess as to what viruses are most likely to be included in the vaccine. For information about flu vaccine production, see How Influenza (Flu) Vaccines Are Made.

Collaborative efforts in the United States across the federal government and the private sector over the past 9 years have led to improved influenza vaccine technologies that have either expanded vaccine supply or improved vaccine effectiveness and in some cases accomplished both of these goals.

Much of the work to improve influenza vaccine technology in the past 9 years has taken place under the auspices of influenza pandemic preparedness planning which is led by the U.S. Department of Health and Human Services’ Biomedical Advanced Research and Development Authority (BARDA). BARDA is charged with the advanced development and procurement of medical and non-pharmaceutical countermeasures for pandemic influenza preparedness and response, including flu vaccines. The Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA) and the National Institutes of Health (NIH) are partners in this broad inter-agency government effort.

As the nation’s leading public health agency, CDC’s primary role in this effort has been to provide the recommendations for the best public health use of existing influenza vaccines. CDC also plays a leading role in conducting surveillance for, helping to select, and producing vaccine viruses used in flu vaccine production. The agency also supports the development of new and better vaccines. For example, CDC’s Influenza Division is using new advanced molecular detection (AMD) and genetics technologies to develop better H3N2 vaccine viruses (that grow well in eggs and that create a good immune response in people). In addition, CDC monitors and reports on the effectiveness of existing influenza vaccines. FDA is the federal regulatory agency responsible for assuring the safety, effectiveness, quality, and security of drugs, vaccines and other biological products and medical devices. NIH is the nation’s medical research agency, a role which includes conducting clinical trials for vaccines.

Many advances have been made with flu vaccines in recent years:

  • a high dose vaccine that is designed specifically for people 65 and older to create a stronger antibody response;
  • a trivalent flu vaccine made with adjuvant (an ingredient added to vaccine that helps create a stronger immune response in the patient’s body), was approved for people 65 years of age and older;
  • an intradermal vaccine that requires less antigen for an equivalent immune response thus stretching vaccine supply;
  • the first U.S.-approved cell-based flu vaccine, which can be made more quickly than traditional egg-based vaccines and does not require a large supply of eggs to produce;
  • quadrivalent (four component) flu vaccines that protect against both lineages of influenza B viruses thus offering expanded protection against circulating influenza viruses; and,
  • the first recombinant influenza vaccines, which can be manufactured more quickly than either egg-based or cell-based vaccines and which does not require an egg-grown vaccine virus nor eggs to produce.

A longer term goal for flu vaccines is the development of a single vaccine that would provide safe, effective and long-lasting immunity against a broad spectrum of influenza viruses, both seasonal and novel. A flu vaccine with these qualities is often referred to as a “universal flu vaccine.”

At this time, CDC is participating in a broad inter-agency partnership coordinated by BARDA that supports the advanced development of new and better influenza vaccines. These efforts already have yielded important successes. But part of this effort is the eventual development of a “universal vaccine” that would offer better, broader and longer-lasting protection against seasonal influenza viruses as well as novel influenza viruses. This task poses an enormous scientific and programmatic challenge, but a number of government agencies and private companies already have begun work to advance development of a universal flu vaccine.

More information on this topic is available at:

There are many different flu viruses and they are constantly changing. The composition of U.S. flu vaccines is reviewed annually and updated as needed to match circulating flu viruses. Flu vaccines protect against the three or four viruses (depending on the vaccine) that research suggests will be most common.

For 2020-2021, trivalent (three-component) egg-based vaccines are recommended to contain:

  • A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus (updated)
  • A/Hong Kong/2671/2019 (H3N2)-like virus (updated)
  • B/Washington/02/2019 (B/Victoria lineage)-like virus (updated)

Quadrivalent (four-component) egg-based vaccines, which protect against a second lineage of B viruses, are recommended to contain:

  • the three recommended viruses above, plus B/Phuket/3073/2013-like (Yamagata lineage) virus.

For 2020-2021, cell- or recombinant-based vaccines are recommended to contain:

  • A/Hawaii/70/2019 (H1N1)pdm09-like virus (updated)
  • A/Hong Kong/45/2019 (H3N2)-like virus (updated)
  • B/Washington/02/2019 (B/Victoria lineage)-like virus (updated)
  • B/Phuket/3073/2013-like (Yamagata lineage) virus

Yes, this season’s flu vaccines were updated to better match viruses expected to be circulating in the United States.

  • The egg-based H1N1 vaccine component was updated from an A/Brisbane/02/2018 (H1N1)pdm09-like virus to an A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus.
  • The cell- or recombinant-based H1N1 vaccine component was updated from an A/Brisbane/02/2018 (H1N1)pdm09-like virus to an A/Hawaii/70/2019 (H1N1)pdm09-like virus.
  • The egg-based H3N2 vaccine component was updated from an A/Kansas/14/2017 (H3N2)-like virus to an A/Hong Kong/2671/2019 (H3N2)-like virus.
  • The cell- or recombinant-based H3N2 vaccine component was updated from an A/Kansas/14/2017 (H3N2)-like virus to an A/Hong Kong/45/2019 (H3N2)-like virus.
  • The B/Victoria lineage vaccine component was updated from a B/Colorado/06/2017 (B/Victoria lineage)-like virus to a B/Washington/02/2019 (B/Victoria lineage)-like virus.
  • The B/Yamagata lineage vaccine component was not updated.

There are two new vaccines licensed for use during the 2020-2021 flu season.

  • The first is a quadrivalent high-dose vaccine licensed for use in adults 65 years and older. This vaccine will replace the previously licensed trivalent high-dose vaccine.
  • The second new vaccine that will be available is a quadrivalent adjuvanted vaccine licensed for use in adults 65 years and older.
    • This vaccine is similar to the previously licensed trivalent vaccine containing MF59 adjuvant, but it has one additional influenza B component.

    More information about new vaccines available this year.

For the 2020-2021 flu season, providers may choose to administer any licensed, age-appropriate flu vaccine (IIV, RIV4, or LAIV4) with no preference for any one vaccine over another.

Vaccine options this season include:

Flu vaccine is produced by private manufacturers, so supply depends on manufacturers. For the 2020-2021 season, manufacturers have projected they will provide as many as 194-198 million doses of flu vaccine, which is more than the 175 million dose record set during the 2019-2020 flu season.

Vaccine Benefits

Public health researchers measure how well flu vaccines work through different kinds of studies. In “randomized studies,” flu vaccination is randomly assigned, and the number of people who get flu in the vaccinated group is compared to the number that get flu in the unvaccinated or placebo group. Randomized studies are the “gold standard” (best method) for determining how well a vaccine works. The effects of vaccination measured in these studies is called “efficacy.” Randomized, placebo controlled studies are expensive and are not conducted after a recommendation for vaccination has been issued, as withholding vaccine from people recommended for vaccination would place them at risk for infection, illness and possibly serious complications. For that reason, most U.S. studies conducted to determine the benefits of flu vaccination are “observational studies.”

“Observational studies” compare the occurrence of flu illness in vaccinated people compared to unvaccinated people, based on their decision to be vaccinated or not. This means that vaccination of study subjects is not randomized. The measurement of vaccine effects in an observational study is referred to as “effectiveness.”

There are many reasons to get an influenza (flu) vaccine each year.
Below is a summary of the benefits of flu vaccination and selected scientific studies that support these benefits. Flu vaccination can keep you from **getting sick with flu.**
  • Flu vaccine prevents millions of illnesses and flu-related doctor’s visits each year. For example, during 2018-2019, flu vaccination prevented an estimated 4.4 million influenza illnesses, 2.3 million influenza-associated medical visits, 58,000 influenza-associated hospitalizations, and 3,500 influenza-associated deaths.
  • During seasons when the flu vaccine viruses are similar to circulating flu viruses, flu vaccine has been shown to reduce the risk of having to go to the doctor with flu by 40 percent to 60 percent.

Flu vaccination can **reduce the risk of flu-associated hospitalization** for children, working age adults, and older adults.

  • Flu vaccine prevents tens of thousands of hospitalizations each year. For example, during 2018-2019 flu vaccination prevented an estimated 58,000 flu-related hospitalizations.
  • 2014 study showed that flu vaccine reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74% during flu seasons from 2010-2012.
  • In recent years, flu vaccines have reduced the risk of flu-associated hospitalizations among older adults on average by about 40%.
  • 2018 study showed that from 2012 to 2015, flu vaccination among adults reduced the risk of being admitted to an intensive care unit (ICU) with flu by 82 percent.

Flu vaccination is an **important preventive tool for people with chronic health conditions.**

  • Flu vaccination has been associated with lower rates of some cardiac events among people with heart disease, especially among those who had had a cardiac event in the past year.
  • Flu vaccination can reduce worsening and hospitalization for flu-related chronic lung disease, such as in persons with chronic obstructive pulmonary disease (COPD).
  • Flu vaccination also has been shown in separate studies to be associated with reduced hospitalizations among people with diabetes and chronic lung disease.

Flu vaccination helps **protect women during and after pregnancy**.

  • Vaccination reduces the risk of flu-associated acute respiratory infection in pregnant women by about one-half.
  • 2018 study that included influenza seasons from 2010-2016 showed that getting a flu shot reduced a pregnant woman’s risk of being hospitalized with flu by an average of 40 percent.
  • A number of studies have shown that in addition to helping to protect pregnant women, a flu vaccine given during pregnancy helps protect the baby from flu for several months after birth, when he or she is not old enough to be vaccinated.

Flu vaccine can be **lifesaving in children.**

  • A 2017 study was the first of its kind to show that **flu vaccination can significantly reduce a child’s risk of dying from flu**.

Flu vaccination has been shown in several studies to **reduce severity of illness in people who get vaccinated but still get sick**.

  • A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized flu patients.
  • 2018 study showed that among adults hospitalized with flu, vaccinated patients were 59 percent less likely to be admitted to the ICU than those who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.

Getting vaccinated yourself may also protect people around you, including those who are more vulnerable to serious flu illness, like babies and young children, older people, and people with certain chronic health conditions.Despite the many benefits offered by flu vaccination, only about half of Americans get an annual flu vaccine and flu continues to cause millions of illnesses, hundreds of thousands of hospitalizations and tens of thousands of deaths. Many more people could be protected from flu if more people got vaccinated.*References for the studies listed above can be found at Publications on Influenza Vaccine Benefits. Also, download the A Strong Defense Against Flu: Get Vaccinated! [635 KB, 2 Pages] fact sheet.

Vaccine Match

A “good match” is said to occur when the flu vaccine viruses used to produce flu vaccine and the viruses circulating among people during a given influenza season are “like” one another such that the antibodies produced by vaccination protect against infection with circulating viruses.

During seasons when one or more of the circulating viruses are different or “drifted” from the vaccine viruses, vaccine effectiveness against the drifted viruses can be reduced. It’s important to remember that flu vaccine protects against three or four different flu viruses and multiple viruses usually circulate during any one season. Even if the effectiveness of the vaccine is reduced against one virus it can still be effective at preventing flu illness caused by the other circulating viruses. For these reasons, CDC continues to recommend flu vaccination for everyone 6 months and older even if vaccine effectiveness against one or more viruses is reduced.

Flu viruses are constantly changing (called “antigenic drift”) – they can change from one season to the next or they can even change within the course of one flu season. Experts must pick which viruses to include in the vaccine many months in advance in order for vaccine to be produced and delivered on time. (For more information about the vaccine virus selection process visit Selecting the Viruses in the Influenza (Flu) Vaccine.) Because of these factors, there is always the possibility of a less than optimal match between circulating viruses and the viruses used to produce vaccine.

The production process for some seasonal vaccines also may impact how well vaccine works against certain viruses, especially influenza A (H3N2) viruses. Growth in eggs is part of the production process for most seasonal flu vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses are more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called “egg-adapted changes” are present in most of the vaccine viruses recommended for use in egg-based vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Advances in vaccine production technologies (for example, cell-based and recombinant technology) and advanced molecular techniques are being explored as ways to improve flu vaccine effectiveness. Learn more by visiting, Advancements in Influenza Vaccines.

It’s not possible to predict with certainty if a flu vaccine will be like circulating flu viruses because flu viruses are constantly changing. A flu vaccine is made to protect against the flu viruses that research and surveillance indicate will likely be most common during the season. Over the course of flu season, CDC studies samples of circulating flu viruses to evaluate how close a match there is between viruses used to make the flu vaccine and circulating flu viruses. More information about the 2019-2020 flu season and recommended vaccines is available.

Vaccine Supply and Distribution

Flu vaccine is produced by private manufacturers, so supply depends on manufacturers. Vaccine manufacturers have projected that they will supply as many as 194 to 198 million doses of influenza vaccine for the 2020-2021 season. These projections may change as the season progresses. Most of this will be quadrivalent vaccine (99%) and thimerosal-free or reduced vaccine (87%). About 20% of flu vaccines will be egg-free.

Information about vaccine supply is available on CDC’s Vaccine Supply & Distribution.

The challenges associated with the U.S. influenza vaccine supply are multi-faceted. Influenza viruses change from year to year, so influenza vaccines must be updated annually to include the viruses that will most likely circulate in the upcoming season. Once the viruses are selected for the new formulation, manufacturers operate under a very tight timeline for producing, testing, releasing and distributing the vaccine. Due to these time constraints, any problems encountered during production may cause shortages or delays, and in fact, such problems have impacted the supply in prior influenza seasons.

Beginning in 2004, CDC began purchasing a late-season influenza vaccine stockpile to provide a limited quantity of vaccine for children (using Vaccines for Children [VFC] Program funds) to address urgent supply problems that may exist in December, January or beyond. Since in most years, influenza disease activity does not peak until January or later, this approach of stockpiling can be quite useful if vaccine production delays or shortages drive demand later into the influenza vaccination season.

Demand for influenza vaccine varies from year to year. Within each influenza vaccination season, demand varies by month, usually peaking in October or November and rapidly declining after that. Meanwhile, because timing of influenza vaccine production and distribution is unpredictable, availability of the vaccine supply does not always coincide with peak demand. Thus, manufacturers with vaccine coming off the production line in middle or late November or later may not be able to sell it all and providers receiving vaccine in this same time frame may not be able to convince patients to receive it, even though late season vaccination is encouraged and mostly beneficial.

At the present time, all six manufacturers licensed in the United States produce at least one formulation that is thimerosal-free or preservative-free (trace thimerosal). Table: Influenza Vaccine Manufacturers for the 2020-21 Influenza Season is available.

Influenza vaccine production and distribution in the US are primarily private sector endeavors. CDC does not have the authority, resources or relationships with providers necessary to carry out or control vaccine distribution.

CDC encourages influenza vaccine manufacturers and distributors to use a distribution strategy that provides vaccine to all provider types in a comparable timeframe. Such an approach allows the broadest possible access for vaccine throughout the season and is consistent with national data that indicate adults in the United States receive influenza vaccine in a wide variety of venues, including private provider offices, worksites, clinics, hospitals, health departments, retail settings, and senior centers. While there is a relationship between age or health status and the location in which vaccines are received, the data demonstrate that all of these venues serve at least some elderly or high risk patients.

The timing of the completion of influenza vaccine production varies from year to year and depends upon on a number of factors, including the strains chosen for inclusion in the vaccine. Even in a year in which each step of production goes well, since influenza vaccine manufacturers are currently producing vaccine at or near full capacity, it isn’t possible for all of the doses to be produced and distributed before the beginning of the vaccination season.

To allow as many providers as possible to begin vaccinating early in the season, CDC encourages manufacturers and distributors to use a distribution strategy in which partial shipments are used to allow as many providers as possible to begin vaccination activities early in the vaccination season. Ideally, the intervals between shipments are short so that each provider has a continuous supply and can continue vaccinating patients without interruption.

Manufacturers say that, because of the unpredictable nature and complexity of biologics production, they cannot always anticipate when vaccine lots will be completed and released. Distributors must rely on the manufacturers to provide them with this information. Both manufacturers and distributors are reluctant to project very far into the future about when shipments will be made because they do not want to create unrealistic expectations among customers.

The primary reason for the variation in timing is because different types of vaccine providers receive their vaccine from different sources. A provider may order influenza vaccine (1) directly from the manufacturer; (2) from a distributor, who is a customer of the manufacturer; or (3) from a secondary distributor, from whom the provider also receives other medical products. Some of these routes of distribution are more direct than others, which can affect the timing of vaccine delivery.

Influenza vaccine production begins as early as 6 to 9 months before the beginning of vaccine distribution. Even with this early start, it isn’t possible to complete the entire production and distribution process prior to flu season, in part because of the limited number of influenza vaccine manufacturing plants in the United States and the large number of doses that are produced each year. Instead, influenza vaccine distribution takes place in a phased fashion over a number of months. Distribution usually begins in late summer and is complete near the end of November or early in December. This may result in some uncertainty making it difficult for vaccine providers to plan their vaccination activities. Manufacturers and distributors try to get some vaccine to as many providers as possible as early as possible so that they can begin vaccinating their patients.

Influenza vaccine production and distribution are primarily private sector endeavors. The Department of Health and Human Services and CDC do not have the authority to control influenza vaccine distribution nor the resources to manage such an effort. However, the Department has made significant efforts to enhance production capacity of seasonal influenza vaccines, including supporting manufacturers as they invest in processes to stabilize and increase their production capacity.

For the upcoming flu season, flu vaccination will be very important to reduce flu because it can help reduce the overall impact of respiratory illnesses on the population and thus lessen the resulting burden on the healthcare system during the COVID-19 pandemic.

For the 2020-2021 season, manufacturers will produce influenza vaccines that do not contain thimerosal and some vaccines that do contain thimerosal. For the 2020-2021 season, only multidose vial presentations of influenza vaccines contain thimerosal.

Approximately 87% of projected vaccine supply, or 171 million doses of flu vaccine, produced for the 2020-2021 flu season will be thimerosal-free or thimerosal-reduced (i.e., preservative-free).

For the 2020-2021 season, manufacturers will produce mostly quadrivalent (four-component) influenza vaccines.

Approximately 99% of the projected vaccine supply produced for the 2020-2021 flu season will be quadrivalent (4-component) vaccines. The only trivalent vaccine available for the 2020-2020 flu season will be the trivalent formulation of the adjuvanted flu vaccine licensed for use in adults 65 years and older.

Approximately 81% of the projected vaccine supply produced for the 2020-2021 flu season will be produced using egg-based manufacturing technology. The remaining vaccine will be produced using cell-based and recombinant technology.

Influenza vaccine pre-booking typically occurs between January and March, though most preparations of vaccine should still be available for purchase. Providers should contact distributors and local vendors about remaining supply. In addition, beginning in early October each year, information about manufacturers and distributors who still have influenza vaccine available for sale can be found.

Updates on the distribution of influenza vaccine doses for the 2020-2021 season will be provided as the season progresses.

The timing of vaccine availability depends on when production is completed. Some influenza vaccine shipments have already begun and will continue throughout August, September, October, and November until all of the vaccine is distributed.