No medical therapy is 100% effective, though the HPV vaccine is pretty close. I used to get the flu periodically, as many people do. In the 10+ years since I've been getting the flu vaccine, I only had the flu once: H1N1, for which I hadn't gotten a vaccine. I'm much happier not getting the flu every (approximately) third winter. I'm also pretty sure I haven't infected anyone else with the flu since I haven't been infected with it myself.