I get the flu shot every year, and like clockwork, about six hours after I get aches, a fever, and shivers. If I pre-emptively take Tylenol it's usually fine. And even if I don't, it's gone the next day. So technically, I'm not sick, but my body reacts as if I am.

It's no reason to not get a flu shot, but I do get a little annoyed when people say it's impossible to get sick from it, without providing that caveat, because then people are more likely to feel deceived if they have a mild immune response and distrust vaccines.