This seems to come up a lot in other threads, so maybe it deserves a thread of it's own, anyway. I came across this article in the New England Journal of Medicine that does a good job summarizing why outbreaks often get "blamed" on unvaccinated children.
Basically it says that outbreaks tend to be correlated geographically with areas where there is a larger population of unvaccinated people and a greater proportion than expected of people who got sick were infected by contact with a non-vaccinated person (I don't really know how they can tell this, I'm just saying that's what it says). For example, in a community if 4% of people are unvaccinated we would expect about 4% of cases of whatever disease to be attributed to contact with an unvaccinated person. If, in fact, 11% are that's a clue that they're infecting people at a higher than expected rate.
Anyway, I know lots of people will disagree, take it FWIW, I just thought people might e interested in where that claim comes from and what the data behind it is like.