The flu vaccine The flu vaccine is the best way to prevent the flu and its complications. Even if the vaccine does not entirely prevent the flu, it may reduce the risk of serious illness requiring hospitalization. Last yearâs flu vaccine was 45% effective. The Centers for Disease Control and Prevention (CDC) recommends children 6 months or older receive a flu vaccine every year in the fall, usually by the end of October. There are situations where children may need two doses for the best protection. But do not worry; getting the vaccine later is better than not getting it at all. Flu season can last well into spring. So, if flu viruses are still circulating, it is never too late to be vaccinated.