Flu viruses usually cause the most illness during the colder months of the year. In the United States, flu season is from October to May. Most cases happen from late December to early March.
It's best to get the flu vaccine early in flu season, ideally by the end of October. That way, the body has time to make antibodies that protect it from the flu.
What if you aren't vaccinated by then? Getting the vaccine later is better than not getting it at all. It's still flu season well into spring. Even then it's not too late for you and your family to get the flu vaccine. Many health care providers give flu vaccines through May if the flu virus is still circulating.
Getting a missed flu vaccine late in the season is especially important for people who travel. That's because the flu can be active around the globe from April to September.