If vaccination really was effective, there would be no risk from the un-vaccinated...
Common myth. Actually, the more a virus has a chance to incubate and spread, the more it mutates posing risks to those who were vaccinated. (think of the flu virus' constant mutation). During the early days of TB vaccination, for example, they were dealing with many strains evolving and had trouble staying ahead of it. Now that most have been vaccinated with a 'group' vaccination, the virus isn't spreading and mutating as much so existing vaccines remain effective. If more and more are unvaccinated and contract it, the more it mutates and spreads potentially rendering existing vaccinations useless.
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There are many children that cannot be vaccinated, for various reasons such as an autoimmune disease, allergies, or simply being too young to have received the vaccine. These children have no protection against the disease. If they are exposed to it through an unvaccinated peer, they are at risk of suffering and/or even dying.
Unvaccinated children are protected by the herd immunity created by the vaccinated children. Herd immunity basically means that if enough people are vaccinated it becomes really hard for the disease to find hosts it can survive in and spread. The more children are unvaccinated the greater the risk that herd immunity will fail. If herd immunity fails, all suffer for the reasons described below.
Vaccines do not offer 100% immunity towards disease. The efficacy varies; some vaccines offer higher rates of efficacy, some lower. Having received a vaccine doesn’t guarantee that a child will not get sick when exposed to the disease. Vaccines reduce the risk of contracting the disease, if exposed, dramatically, but there will always be a number of children for whom the vaccine will not provide protection. Those children will be at risk, from other unvaccinated children who may contract and spread the disease.