How do strains of influenza differ?
• Strains of influenza differ based upon combinations of certain proteins on the surface of the virus. • With human strains, minor changes, called drift, occur each influenza season. • Major changes, called shift, also can occur, and these strains tend to cause larger outbreaks of flu. • The influenza virus changes enough each year to make most people susceptible to getting the flu. • Vaccine manufacturers modify the seasonal influenza shot each year to reflect the changes in the circulating virus. • In a typical influenza season, several strains will be circulating.
