They can't explain. They don't know enough to try. They just know that as long as he's human — which feels weird to stipulate, since they're only recently becoming aware that nonhumans exist among them — then he needs this shot.

I just took a peek at your application to confirm, and he definitely would not shrug off what they are attempting to give him a shot to prevent. The consequences can be reversed at any time, but it would have to be an IC choice on his part unless it came to be an actual danger to others.