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Flu Vaccine: Key Facts

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 serious disease that can lead to hospitalization and sometimes even death. Every flu season is different, and influenza infection can affect people differently. Even healthy people can get very sick from the flu and spread it to others. Over a period of 31 seasons between 1976 and 2007, estimates of flu-associated deaths in the United States range from a low of about 3,000 to a high of about 49,000 people. During recent flu seasons, between 80% and 90% of flu related deaths have occurred in people 65 years and older.

"Flu season" in the United States can begin as early as October and last as late as May. During this time, flu viruses are circulating at higher levels in the U.S. population. An annual seasonal flu vaccine (either the flu shot or the nasal spray flu vaccine) is the best way to reduce the chances that you will get seasonal flu and spread it to others. When more people get vaccinated against the flu, less flu can spread through that community.

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 injectable influenza vaccines (including inactivated influenza vaccines and recombinant influenza vaccines) during 2016-2017. The nasal spray flu vaccine (live attenuated influenza vaccine or LAIV) should not be used during 2016-2017. 

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

Trivalent flu vaccines include:

  • Standard-dose trivalent shots (IIV3) that are manufactured using virus grown in eggs. Different flu shots are approved for different age groups. Most flu shots are given in the arm (muscle) with a needle. One trivalent vaccine formulation can be given with a jet injector, for persons aged 18 through 64 years.
  • A high-dose trivalent shot, approved for people 65 and older.
  • A recombinant trivalent shot that is egg-free, approved for people 18 years and older.
  • A trivalent flu shot made with adjuvant (an ingredient of a vaccine that helps create a stronger immune response in the patient’s body), approved for people 65 years of age and older (new this season).

Quadrivalent flu vaccines include:

  • Quadrivalent flu shots approved for use in different age groups.
  • An intradermal quadrivalent flu shot, which is injected into the skin instead of the muscle and uses a much smaller needle than the regular flu shot. It is approved for people 18 through 64 years of age.
  • A quadrivalent flu shot containing virus grown in cell culture, which is approved for people 4 years of age and older (new this season).  

Are any of the available flu vaccines recommended over others?

For the 2016-2017 flu season, the Advisory Committee on Immunization Practices (ACIP) recommends annual influenza vaccination for everyone 6 months and older with either the inactivated influenza vaccine (IIV) or the recombinant influenza vaccine (RIV). The nasal spray flu vaccine (live attenuated influenza vaccine or LAIV) should not be used during 2016-2017.There is no preference for one vaccine over another among the recommended, approved injectable influenza vaccines. 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 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.

Who Should Not Be Vaccinated?

CDC recommends use of the flu shot (inactivated influenza vaccine or IIV) and the recombinant influenza vaccine (RIV).

The nasal spray flu vaccine (live attenuated influenza vaccine or LAIV) should not be used during 2016-2017.

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 sometimes updated 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 early in the fall, before the flu season really gets under way.

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

Yes. There is still a possibility you could get the flu even if you got vaccinated. The ability of flu vaccine to protect a person depends on various factors, including the age and health status of the person being vaccinated, and also the similarity or "match" between the viruses used to make the vaccine and those circulating in the community. If the viruses in the vaccine and the influenza viruses circulating in the community are closely matched, vaccine effectiveness is higher. If they are not closely matched, vaccine effectiveness can be reduced. However, it's important to remember that even when the viruses are not closely matched, the vaccine can still protect many people and prevent flu-related complications. Such protection is possible because antibodies made in response to the vaccine can provide some protection (called cross-protection) against different but related influenza viruses.

Vaccine Effectiveness

Influenza vaccine effectiveness (VE) can vary from year to year and among different age and risk groups. 

Vaccine Benefits

What are the benefits of flu vaccination?

While how well the flu vaccine works can vary, there are a lot of reasons to get a flu vaccine each year.

  • Flu vaccination can keep you from getting sick with flu.
  • Flu vaccination can reduce the risk of flu-associated hospitalization, including among children and older adults.
    • 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.
    • Another study published in the summer of 2016 showed that people 50 years and older who got a flu vaccine reduced their risk of getting hospitalized from flu by 57%.
  • Flu vaccination is an important preventive tool for people with chronic health conditions.
    • Vaccination was 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 to be associated with reduced hospitalizations among people with diabetes (79%) and chronic lung disease (52%).
  • Vaccination helps protect women during and after pregnancy. Getting vaccinated also protects the developing baby during pregnancy and for several months after the baby is born.
    • A study that looked at flu vaccine effectiveness in pregnant women found that vaccination reduced the risk of flu-associated acute respiratory infection by about one half.
    • Another study found that babies of women who got a flu vaccine during their pregnancy were about one-third less likely to get sick with flu than babies of unvaccinated women. This protective benefit was observed for up to four months after birth.
  • Flu vaccination also may make your illness milder if you do get sick.
  • Getting vaccinated yourself also protects 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.

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 viruses in the vaccine 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 from the vaccine viruses, vaccine effectiveness can be reduced.

Can the vaccine provide protection even if the vaccine is not a "good" match?

Yes, antibodies made in response to vaccination with one flu virus can sometimes provide protection against different but related viruses. A less than optimal match may result in reduced vaccine effectiveness against the virus that is different from what is in the vaccine, but it can still provide some protection against influenza illness.

In addition, even when there is a less than optimal match or lower effectiveness against one virus, it's important to remember that the flu vaccine is designed to protect against three or four flu viruses, depending on the vaccine.

For these reasons, even during seasons when there is a less than optimal match, CDC continues to recommend flu vaccination. This is particularly important for people at high risk for serious flu complications, and their close contacts.

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.

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

It's not possible to predict with certainty which flu viruses will predominate during a given season. Over the course of a flu season, CDC studies samples of flu viruses circulating during that season by looking at their genetic and antigenic properties to evaluate how close a match there is between the viruses recommended for vaccine production and circulating viruses. Data are published in the weekly FluView. In addition, CDC conducts studies each year to determine how well the vaccine protects against illness during that season. The results of these studies are typically published following the conclusion of the flu season and take into consideration all of the data collected during the season. Interim preliminary estimates of the vaccine's benefits that season using data available at that time also may be provided.

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 'inactivated' and are therefore not infectious, or b) with no flu vaccine viruses at all (which is the case for recombinant influenza vaccine). The nasal spray flu vaccine does contain live viruses. 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.

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:

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:

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

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).


Reference: Centers for Disease Control and Prevention

Last updated September 22, 2016