Growing up, I knew a young man who'd lost his hearing as a result of the smallpox vaccine. (He could play chess and speak Latin, two things I have never learned to do, so I was impressed by him.) Of course smallpox could disable too, and kill.
He had an allergic reaction to the vaccine. Allergies happen. There are people who die from eating peanuts, or honey. You should hear the legalize marijuanna people when I tell them I'm allergic to the stuff.
But you might want to introduce your stepson to the history of Jenner and the smallpox vaccine. It's an interesting story all by itself.