It doesn't. The diseases that are immunized against mutate extremely slowly, and so few people have them anymore that the disease doesn't proliferate enough to have a chance to do so.
US teen who defied parents over vaccine warns of misinformation
Charles Walker
Xavier Russell
the people who were sharing stories of children dying or developing autism were paid out to stop speaking against vaccines and they all came out and said they regretted accepting bribes to not share their stories anymore.