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 in a given year. About a fortnight after vaccination, antibodies that provide protection against influenza virus infection develop in the body.

The seasonal flu vaccine will not provide protection against 2009 H1N1 flu.