each year the seasonal influenza vaccine contains three influenza viruses – one influenza a (h3n2) virus one regular seasonal influenza a (h1n1) virus (not the 2009 h1n1 virus) and one influenza b virus. the influenza viruses in the seasonal flu vaccine are selected each year based on (surveillance-based) forecasts about what viruses are most likely to cause illness in the coming season. therefore each year’s seasonal flu vaccine is designed to protect against the seasonal influenza viruses expected to cause disease during that influenza season. great question!
what flu strain is the seasonal vaccine?