Each vaccine contains three influenza viruses-one A (H3N2) virus, one A (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.

For information concerning prevention and control please visit the CDC website.