I always thought the theory of herd immunity is that once you get everyone vaccinated that can be, it protects those who cant be.
Its when people start objecting and not vaccinating for no good reason that rates drop too low to protect the vulnerable. Thats why whenever there's outbreaks of measles etc they are usually in the areas with the lowest vaccination rates.
Vaccination acts as a sort of firebreak or firewall in the spread of the disease, slowing or preventing further transmission of the disease to others.[3] Unvaccinated individuals are indirectly protected by vaccinated individuals, as the latter are less likely to contract and transmit the disease between infected and susceptible individuals.[2] Hence, a public health policy of herd immunity may be used to reduce spread of an illness and provide a level of protection to a vulnerable, unvaccinated subgroup. Since only a small fraction of the population (or herd) can be left unvaccinated for this method to be effective, it is considered best left for those who cannot safely receive vaccines because of a medical condition such as an immune disorder, organ transplant recipients, or people with egg allergies.
If every child that could be safely vaccinated was, herd immuity would be more effective and we might very well wipe out a few more of the diseases. As long as there are people like the example earlier (milk and onion I think it was) those that cant be vaccinated are at much higher risk.
