Key Facts About Seasonal Flu Vaccine
The single best way to protect against the flu is to get vaccinated each year.
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.
CDC recommends use of any licensed, age-appropriate influenza vaccine during the 2018-2019 influenza season, including inactivated influenza vaccine [IIV], recombinant influenza vaccine [RIV], or live attenuated influenza vaccine (LAIV). No preference is expressed for any influenza vaccine over another. Both trivalent (three-component) and quadrivalent (four-component) flu vaccines will be available.
Trivalent flu vaccines include:
- One standard-dose trivalent flu shot (IIV3) manufactured using virus grown in eggs. This shot (Afluria) can be given either with a needle (for people aged 5 years and older) or with a jet injector (or people aged 18 through 64 years only).
- A high-dose trivalent flu shot (Fluzone High-dose), approved for people 65 years and older.
- A trivalent flu shot made with adjuvant (Fluad), approved for people 65 years and older.
Quadrivalent flu vaccines include:
- Standard-dose quadrivalent flu shots that are manufactured using virus grown in eggs. These include Afluria Quadrivalent, Fluarix Quadrivalent, FluLaval Quadrivalent, and Fluzone Quadrivalent. Different flu shots 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 (Afluria Quadrivalent) can be given either with a needle (for people aged 5 years and older) or with a jet injector (for people aged 18 through 64 years only).
- A quadrivalent cell-based flu shot (Flucelvax Quadrivalent) containing virus grown in cell culture (that is egg-free), which is approved for people 4 years and older.
- A recombinant quadrivalent flu shot (Flublok Quadrivalent) approved for people 18 years and older.
Are any of the available flu vaccines recommended over others?
For the 2018-2019 flu season, the Advisory Committee on Immunization Practices (ACIP) recommends annual influenza vaccination for everyone 6 months and older with any licensed age-appropriate flu vaccine including inactivated influenza vaccine (IIV), recombinant influenza vaccine (RIV4) or live attenuated influenza vaccine (LAIV4) with no preference expressed for any one vaccine over another.
Who should get vaccinated this season?
Everyone 6 months of age and older should get a flu vaccine every season. This recommendation has been in place since February 24, 2010 when CDC’s Advisory Committee on Immunization Practices (ACIP) voted for “universal” flu vaccination in the United States to expand protection against the flu to more people.
Vaccination to prevent influenza is particularly important for people who are at high risk of serious complications from influenza. 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 flu vaccines are approved for use in different age groups. In addition, some vaccines are not recommended for certain groups. 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 begins spreading in your community. It takes about two weeks after vaccination for antibodies that protect against flu to develop in the body. CDC recommends that people get a flu vaccine by the end of October. Getting vaccinated later, however, can still be beneficial and 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.
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 Finder 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.Top of Page
Influenza vaccine effectiveness (VE) can vary from year to year. 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 or virus in the vaccine and those in circulation. For more information, see Vaccine Effectiveness – How well does the Flu Vaccine Work.
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.Top of Page
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 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 2016-2017, flu vaccination prevented an estimated 85,000 flu-related hospitalizations.
- 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 recent years, flu vaccines have reduced the risk of flu-associated hospitalizations among adults on average by about 40%.
- A 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.
- 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 also has been shown in separate studies to be associated with reduced hospitalizations among people with diabetes and chronic lung disease.
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-half.
- 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 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.
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.Top of Page
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)
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
- Muscle aches
In adults, side effects from the nasal spray vaccine may include:
- Runny nose
- Sore throat
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).
More information about the safety of flu vaccines is available at Influenza Vaccine Safety.Top of Page
How much influenza vaccine is projected to be available for the 2018-2019 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 163 million to 168 million doses of i flu vaccine for the 2018-2019 season.
Where can I find information about vaccine supply?
Information about vaccine supply is available on the CDC’s Vaccine Supply & Distribution web site.Top of Page
People with egg allergies can receive any licensed, recommended age-appropriate influenza 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.
- Page last reviewed: September 6, 2018
- Page last updated: September 6, 2018
- Content source:
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
- Page maintained by: Office of the Associate Director for Communication, Digital Media Branch, Division of Public Affairs