Flu shot takes about two weeks before it becomes effective.
We'd always been told end of October or beginning of November in the past because flu seemed to hit around Thanksgiving when lots of people travel and to carry us thru to March/April; however, it seems as if the flu season has started earlier and earlier. Seems to hit our area particularly hard mid-October. Add to that, the local clinics seem to have underestimated the need and run out almost as soon as it becomes available.
Last year we were told to get vaccinated as soon as the shot became available.