The best time to get a flu shot is right after vaccines become available in the fall, usually by October. However, the flu vaccine typically remains available through the entire flu season, even going into January or February, according to the Centers for Disease Control and Prevention.
After vaccination, the body takes about two weeks to develop the antibodies that provide protection from infection with the influenza virus. Most seasonal outbreaks hit a peak in January or later, but outbreaks can begin in October. For this reason, getting vaccinated before outbreaks begin provides optimal protection, as stated by the CDC.