So very sorry to hear about your brother. I have a younger relative who is just now joining the ranks of commercial pilots and I’m worried about him given that the shot(s) were a precondition of employment.
It is a game of percentages... he will most likely be fine. Our bodies are amazing and can overcome a lot of idiotic nonsense. But every once in a while something will trip them up.
I do not believe that public safety is the real reason for these shots.