Widespread vaccination coverage among children is responsible for reducing or eliminating 14 serious childhood diseases in the United States. Despite this success, some parents remain concerned about the health effects of vaccines, and choose to keep their children unvaccinated. When population rates of vaccinations remain high enough, even unvaccinated children are protected because everyone around them is immune (so-called “herd immunity”). But clusters of unvaccinated children may threaten herd immunity and lead to increasing outbreaks of preventable diseases. This Issue Brief summarizes a new study that investigates the extent of such clustering, and quantifies the exposure of all California kindergartners to their intentionally unvaccinated schoolmates.