In BC, flu shots have historically only been free for those with health issues, those who live with them, and I guess the elderly probably. BC always left working ages adults with no recorded health issues out to dry as far as I can tell.
Edit: HISTORICALLY guys!
To be fair, in places where income taxes are bracketed rather than in smooth percentages, it can happen. It’s much more common with bonuses and heavy OT to jump you up for a single paycheque, though.