Mandatory Vaccinations

Vaccination boosts resistance, it doesn't confer immunity.
The more susceptible people present the less benefit having an increased resistance does you because the more direct contact/exposure your immune system has to deal with.

Direct exposure is exactly how your immune system builds immunity. So the best way to reduce the problem is to not "fool" the immune system with a vaccine. Vaccines are dead forms of viruses/diseases. However, the immune system only builds a defense against a dead form. If exposed to the living form, the immune system may not be properly prepared for the way the virus/disease acts. Thus causing an infection anyway. Small pox is one example of such occurance, another can be the flu.