I have had probably 15 flu shots over my working career, and then one every year since I retired. I never suffered any illness in the time period after the shot. Never got the flu, either around the time of the shot or afterwards.
A person who gets a flu shot receives dead virus and cannot get the flu from dead virus— because they’re dead and can’t replicate— being dead and all. It takes 2 weeks for the shot to take effect. It is possible to have been exposed around the time of inoculation, prior to becoming immune.
I have read two scientific studies that find that the flu shot isn’t particularly efficacious in seniors, so I will probably stop taking it— even though I get it at no cost through my health plan.
http://cid.oxfordjournals.org/content/early/2012/03/13/cid.cis307
“Increased risk of non-influenza respiratory virus infections associated with receipt of inactivated influenza vaccine”