A new flu vaccine is prepared every year because the strains of flu viruses change from year to year. Nine to 10 months before the flu season begins, scientists prepare a new vaccine made from inactivated (killed) flu viruses. The vaccine preparation is based on the strains of the flu viruses that are in circulation at the time. It includes those A and B viruses expected to circulate the following winter.

Another reason to get vaccinated for the flu every year is that immunity afterwards declines and may be too low to provide protection after one year.