Question: Why is it that when poeple get the injections against measles, mumps and rubella some people still get the disease while others dont?

Peter Balfe answered on 26 Jun 2013:

1 or 2 in a million of us has a natural immune deficiency of some sort. If you immunise 10 million people, you’ll find them. But “some people” is still 1 in a million.

Most symptoms of infection caused by vaccines are the faintest echo of the real thing, but the very mild symptoms occur often because the vaccine is a massively disabled version of the actual disease organism.