Hmmm.... what could it be? Could it be the... nah, couldn’t be THAT. Seriously though, hope he’s okay!
What could it be often has the connotation that there is more than one possibility. Your rather snarky response belies that possibility. Do you agree to the following:
1. We don’t know if he was vaccinated to a certainty.
2. Even were he, there are other conditions which may cause what is occurring.
3. Btw. What is he in the hospital for? The last thread on this issue wasn’t quite sure despite the answer being directly in the article.