Each seasonal influenza vaccine contains three influenza viruses-one A (H3N2) virus, one regular seasonal A (H1N1) virus (not the 2009 H1N1 virus), and one B virus. The viruses in the vaccine change each year based on international surveillance and scientists' estimations about which types and strains of viruses will circulate in a given year. About 2 weeks after vaccination, antibodies that provide protection against influenza virus infection develop in the body.

The seasonal flu vaccine will not provide protection against2009 H1N1 flu.

When to Get Vaccinated

Yearly flu vaccination should begin in September or as vaccine is available and continue throughout the influenza season, into December, January, and beyond. This is because the timing and duration of influenza seasons vary. While influenza outbreaks can happen as early as October, most of the time influenza activity peaks in January or later.

Who Should Get Vaccinated

In general, anyone who wants to reduce their chances of getting seasonal flu can get a seasonal influenza vaccine. However, it is recommended by ACIP that certain people should get vaccinated each year. They are either people who are at high risk of having serious seasonal flu-related complications or people who live with or care for those at high risk for serious seasonal flu-related complications. During flu seasons when vaccine supplies are limited or delayed, ACIP makes recommendations regarding priority groups for vaccination.

People who should get the seasonal vaccine each year are:

1. Children aged 6 months up to their 19th birthday
2. Pregnant women
3. People 50 years of age and older
4. People of any age with certain chronic medical conditions
5. People who live in nursing homes and other long-term care facilities
6. People who live with or care for those at high risk for complications from flu, including:
   1. Health care workers
   2. Household contacts of persons at high risk for complications from the flu
   3. Household contacts and out of home caregivers of children less than 6 months of age (these children are too young to be vaccinated)

   Who Should Not Be Vaccinated

   There are some people who should not get a flu vaccine without first consulting a physician. These include:

   • People who have a severe allergy to chicken eggs.
   • People who have had a severe reaction to an influenza vaccination.
   • People who developedGuillain-Barré syndrome (GBS)within 6 weeks of getting an influenza vaccine.
   • Children less than 6 months of age (influenza vaccine is not approved for this age group), and
   • People who have a moderate-to-severe illness with a fever (they should wait until they recover to get vaccinated.)

   Vaccine Effectiveness

   The ability of a flu vaccine to protect a person 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.

   Vaccine Side Effects (What to Expect)

   Different side effects can be associated with the flu shot and LAIV.

   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 could occur are:

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

   If these problems occur, they begin soon after the shot and usually last 1 to 2 days. 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.