I'm certainly not a doctor, but it's my understanding that vaccines are supposed to get the immune system to fight off whatever it's designed for, whether flu, measles, Covid, etc. We all have immune systems that vary in response to some vaccines and that certain conditions can lead to adverse reactions, even if rare. It makes sense that some people with unusually sensitive immune systems or immuno-compromised would have adverse reactions to a given vaccine. So I don't doubt it happens. I guess the question is how often does that happen and how severe are the reactions.