Key Facts About Seasonal Flu Vaccine

little girl getting a bandaid after a shot

The single best way to protect against the flu is to get vaccinated each year.

Flu Vaccination

Why should people get vaccinated against the flu?

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 the flu every year, hundreds of thousands of people are hospitalized and thousands or 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.

How do flu vaccines work?

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 in the vaccine.

The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season. Traditional flu vaccines (called “trivalent” vaccines) are made to protect against three flu viruses; an influenza A (H1N1) virus, an influenza A (H3N2) virus, and an influenza B virus. There are also flu vaccines made to protect against four flu viruses (called “quadrivalent” vaccines). These vaccines protect against the same viruses as the trivalent vaccine and an additional B virus.

What kinds of flu vaccines are available?

CDC recommends use of any licensed, age-appropriate influenza (flu) vaccine during the 2019-2020 influenza season. Options include inactivated influenza vaccine [IIV], recombinant influenza vaccine [RIV], or live attenuated influenza vaccine (LAIV). Different vaccines are licensed for different age groups, and some vaccines are not recommended for some groups of people. But where more than one suitable vaccine is available, no preference is expressed for any influenza flu vaccine over another.

Both trivalent (three-component) and quadrivalent (four-component) flu vaccines will be available for 2019-2020. Most vaccines will be quadrivalent.

Trivalent flu vaccines include:

  • A high-dose trivalent flu shot, approved for people 65 years and older, which contains a higher dose of antigen (the part of the virus the immune system develops antibodies against), to help create a stronger immune response.
  • A trivalent flu shot made with adjuvant, (an ingredient that helps create a stronger immune response), approved for people 65 years and older.

Quadrivalent flu vaccines include:

  • Standard-dose quadrivalent flu shots without adjuvant that are manufactured using virus grown in eggs. Several different brands of this type of flu shot are available, and they are approved for different age groups. Some are approved for children as young as 6 months of age. Most flu shots are given in the arm (muscle) with a needle. One quadrivalent flu shot can be given either with a needle (for people aged 6 months and older) or with a jet injector (for people aged 18 through 64 years only).
  • A quadrivalent cell-based flu shot containing virus grown in cell culture, which is approved for people 4 years and older.
  • A recombinant quadrivalent flu shot (a flu shot that is made without influenza viruses or eggs) approved for people 18 years and older.
  • A quadrivalent live attenuated influenza nasal spray vaccine (LAIV4) made with attenuated (weakened) live flu viruses, approved for use in people 2 years through 49 years of age. This vaccine is not recommended for use in pregnancy or for use among people with some specific medical conditions.

Are any of the available flu vaccines recommended over others?

For the 2019-2020 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.

Who should get vaccinated this season?

Everyone 6 months of age and older should get an influenza (flu) vaccine every season with rare exception. CDC’s Advisory Committee on Immunization Practices has made this recommendation since the 2010-11 influenza season.

Vaccination to prevent flu is particularly important for people who are at high risk of developing serious flu complications. See People at High Risk of Developing Flu-Related Complications for a full list of age and health factors that confer increased risk.

More information is available at Who Should Get Vaccinated Against Influenza.

Who Should Not Be Vaccinated?

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.

When should I get vaccinated?

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, if possible. Getting vaccinated later, however, can still be beneficial and vaccination should continue to be offered throughout the flu season, even into January or later.

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.

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.

Where can I get a flu vaccine?

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 HealthMap Vaccine Finderexternal icon to locate where you can get a flu vaccine.

Why do I need a flu vaccine every year?

A flu vaccine is needed every season for two reasons. First, the body’s immune response from vaccination declines over time, so an annual vaccine is needed for optimal protection. Second, because flu viruses are constantly changing, the formulation of the flu vaccine is reviewed each year and updated as needed to keep up with changing flu viruses. For the best protection, everyone 6 months and older should get vaccinated annually.

Does flu vaccine work right away?

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 better to get vaccinated by the end of October, before the flu season really gets under way.

Vaccine Effectiveness

Influenza (flu) vaccine effectiveness (VE) can vary. The protection provided by a flu vaccine depends 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. 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.

Can I get seasonal flu even though I got a flu vaccine this year?

Yes. 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:

  • You may be exposed to a flu 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 flu before the vaccine begins to protect you. (Antibodies that provide protection develop in the body about 2 weeks after vaccination.)
  • You may be exposed to a flu virus that is not included in the seasonal flu vaccine. There are many different flu viruses that circulate every year. A flu vaccine is made to protect against the three or four flu viruses that research suggests will be most common.
  • Unfortunately, some people can become infected with a flu virus a flu vaccine is designed to protect against, despite getting vaccinated. Protection provided by flu vaccination can vary widely, based in part on health and age factors of the person getting vaccinated. In general, a flu 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. Flu vaccination is not a perfect tool, but it is the best way to protect against flu infection.

What protection does the flu vaccine provide if I do get sick with flu?

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 flu patients. Another study in 2018 showed that a vaccinated adult who was hospitalized with flu was 59 percent less likely to be admitted to the Intensive Care Unit 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 4 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.

Vaccine Benefits

What are the benefits of flu vaccination?

There are many reasons to get a 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 2016-2017, flu vaccination prevented an estimated 5.3 million influenza illnesses, 2.6 million influenza-associated medical visits, and 85,000 influenza-associated hospitalizations.
    • In seasons when the vaccine viruses matched circulating strains, 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 vaccination helps prevent serious medical events associated with some chronic conditions.
  • Vaccination helps protect women during and after pregnancy.
    • Vaccination reduces the risk of flu-associated acute respiratory infection in pregnant women by up to one-halfexternal icon.
    • A 2018 studyexternal icon showed that getting a flu shot reduced a pregnant woman’s risk of being hospitalized with flu by an average of 40 percent.
    • Getting vaccinated can also protect a baby after birth from flu. (Mom passes antibodies onto the developing baby during her pregnancy.)
      • 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 infection for several months after birth, when he or she is not old enough to be vaccinated.
  • Flu vaccine can be life-saving 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 influenza.
  • 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.
    • A 2018 studyexternal icon 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.

*References for the studies listed above can be found at Publications on Influenza Vaccine Benefits. Also, see the A Strong Defense Against Flu Factsheet pdf icon[237 KB, 2 pages] fact sheet.

Vaccine Match

What is meant by a “good match” between viruses in the vaccine and circulating influenza viruses?

A “good match” is said to occur when the flu viruses the flu vaccine is designed to protect against and the viruses circulating among people during a given influenza season are closely related and the antibodies produced by vaccination protect against infection.

What if circulating viruses and the vaccine viruses are different?

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.

Why is there sometimes not a good match between a vaccine virus and circulating viruses?

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 in the vaccine.

The production process for some seasonal vaccines also can impact the vaccines effectiveness against certain viruses, like 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 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 most of the vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Advances in vaccine production technologies and advanced molecular techniques are being explored as ways to improve flu vaccine effectiveness. Learn more by visiting, Advancements in Influenza Vaccines.

Will this season’s vaccine be a good match for circulating viruses?

It’s not possible to predict with certainty if a flu vaccine will be a good match for circulating flu viruses because flu viruses are constantly changing. More information about the 2018-2019 flu season and recommended vaccines is available.

Vaccine Side Effects (What to Expect)

Can the flu vaccine give me the flu?

No, a flu vaccine cannot cause flu illness. Flu vaccines that are administered with a needle are currently made in two ways: the vaccine is made either with a) flu vaccine viruses that have been killed (inactivated) and are therefore not infectious, or b) with proteins from a flu vaccine virus instead of flu vaccine viruses (which is the case for recombinant influenza vaccine). However, the viruses are attenuated (weakened), and therefore cannot cause flu illness. The weakened viruses are cold-adapted, which means they are designed to only cause infection at the cooler temperatures found within the nose. The viruses cannot infect the lungs or other areas where warmer temperatures exist.

What side effects can occur after getting a flu vaccine?

While a flu vaccine cannot give you flu illness, there are different side effects that may be associated with getting a flu shot or a nasal spray flu vaccine. These side effects are mild and short-lasting, especially when compared to symptoms of bad case of flu.

The flu shot: The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. Some minor side effects that may occur are:

  • Soreness, redness, or swelling where the shot was given
  • Fever (low grade)
  • Aches

The nasal spray: 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

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 are mild and short-lived. Almost all people who receive influenza vaccine have no serious problems from it. However, on rare occasions, flu vaccination can cause serious problems, such as severe allergic reactions. People who think that they have been injured by the flu shot can file a claim for compensation from the National Vaccine Injury Compensation Program (VICP)external icon.

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

Vaccine Supply and Distribution

How much influenza vaccine is projected to be available for the 2019-2020 influenza season?

Flu vaccine is produced by private manufacturers, so supply depends on manufacturers. Vaccine manufacturers have projected that they will supply as many as 162 million to 169 million doses of influenza vaccine for the 2019-2020 season.

Where can I find information about vaccine supply?

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

Special Consideration Regarding Egg Allergy

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.