Prevention

There are two influenza (flu) vaccines. The "flu shot" is an inactivated vaccine (containing killed virus) given with a needle and injected into the muscle. There also is a nasal-spray flu vaccine (sometimes called LAIV for Live Attenuated Influenza Vaccine) that contains weakened live viruses. About two weeks after vaccination, antibodies develop in the body that protect against infection by viruses similar to those in the vaccine.

Each year, the seasonal influenza vaccine contains three influenza virus strains – one influenza A (H3N2) virus, one influenza A (H1N1) virus, and one influenza B virus. The influenza virus strains in the vaccine are selected each year based on (surveillance-based) forecasts about what virus strains are most likely to cause illness in the coming season. Therefore, each year’s vaccine is designed to protect against the influenza viruses expected to cause disease during that influenza season.

The viruses used in making flu vaccine are chosen each year based on information gathered over the previous year about the strains of flu viruses that are infecting humans and how they are changing.

The vaccine will not cause the flu. Its side effects are soreness at the injection site, low-grade fever, sore muscles and fatigue. The vaccine is produced in eggs and should not be given to those with egg allergies.

Vaccine is usually given in October or November, one to two months before the onset of flu season to those in the high-risk categories to allow time for antibody production.