The ideal time to get a flu vaccination is as soon as it becomes available each fall, particularly by October, according to the U.S. Center for Disease Control. Initial supplies of flu vaccines are normally available as early as August each year.
One benefit of early treatment is that it takes two weeks for antibodies to take full effect. The peak of flu activity is normally in January, though, so getting vaccinated by that point is still beneficial, according to the CDC. The CDC encourages anyone 6 months and older to get vaccinated as of 2015. People with high health risks, including the aging population, are highly encouraged to receive vaccination against the flu.