When Should I Get My Flu Shot?
Flu vaccination should begin soon after the vaccine becomes available, ideally by October. However, as long as flu viruses are circulating, vaccination should continue to be offered throughout the flu season, even in January or later. While seasonal influenza outbreaks can happen as early as October, during most seasons influenza activity peaks in January or later. Since it takes about two weeks after vaccination for antibodies to develop in the body that protect against influenza virus infection, it is best that people get vaccinated early so they are protected before influenza begins spreading in their community.
—Source: Centers for Disease Control and Protection