[mzs]

Here is a concrete example. MMR is two dose regiment. The current anre recently updated CDC recommendation is that if a child experiences thrombocytopenia within six weeks after the first dose to not give the second dose. Thrombocytopenia is usually not serious - nose bleeds, easy bruising, bleeding of the gums - but since the risk is higher for those that experienced that side effect after the first dose, that is why it has become the recommendation.

One dose of MMR puts their effective immunity to measles at something like 85% during childhood. It varies with how strong the immune system is when the first dose is given, most children get it at six to nine months. At nine months one dose is 85% effective and at 12 months 95% for most infants. It is presumed that the effect is smaller in those vaccinated at six months and those that experience thrombocytopenia and the recommendation is relying on 95% immunization rate for herd immunity. Something like one in 25K children are in this category. There are other categories of people not immunized as well.