The CDC recommends we all get our flu shot as soon as our doctor or local clinics begin to offer the vaccination, usually in the early autumn, in an effort to protect each of us before flu season officially hits. If you wait until you see headlines about a major flu season underway (or your coworkers and family members start coming down with it), there's a real chance you could be infected before that last-minute shot really starts to protect you. It takes more than a week or two for the benefits of the vaccination to take effect, so there's the possibility that you could get your shot and then develop influenza if you were exposed to the virus either just before or after your vaccination.
So, your odds of preventing the flu from keeping you down increase if you're vaccinated early in the flu season, but the window of opportunity is much larger than you might think. Getting your influenza vaccine sometime between Labor Day and Thanksgiving is considered ideal, but flu season doesn't typically peak until January, February or March -- some seasons it's been known to last through May. And a late shot is better than no shot, so no matter whether it's October or February, the vaccine can still help.