I don't know what a flu shot costs since my insurance pays for that, but that does seem awfully cheap. I'd be wondering where the catch is, but then I'm naturally suspicious, especially about medical things that seem too good to be true.

Edit: Ah! Just saw the above comment. Huh. I wonder if you wouldn't have to explicitly agree to having data harvested like that. Their just doing it without informed consent doesn't sound legally kosher, tbh.