I am not sure, but it could be they use live vaccine...which means the organism is just weakened but not dead, and that can cause problems in some people.

I had hideous chicken pox as a kid and given all my issues, would not doubt that shingles will rear its head....but I will deal with it then...no vaccine for me.

I just had my grandson vaccinated for his 6 month shots, and they gave WAY too many in my estimation. He was miserable for a week. I dunno about all these shots.