Exposure of California kindergartners to students with personal belief exemptions from mandated school entry vaccinations

Objectives: Personal belief exemptions (PBEs) from mandated school entry vaccinations have increased in California over the past decade. Infectious disease outbreaks in the state may be associated with the aggregation of intentionally unvaccinated children within schools. We sought to quantify the exposure of California kindergartners to children with PBEs at school. Methods: We used cross-sectional California Department of Public Health data on 3 kindergarten cohorts to define and calculate multiple measures of exposure to children with exemptions, including interaction and aggregation indices, for the state as a whole (2008-2010) and by county (2010). Results: In 2010, the PBE rate in California was 2.3 per 100 students, and the school PBE rate for the average kindergartner with a PBE was 15.6 per 100. More than 7000 kindergartners in California attend schools with PBE rates greater than 20 per 100, including 2700 kindergartners with PBEs. Exposure measures vary considerably across counties. Conclusions: Our results suggest increasing levels of exposure among kindergarten students in California to other kindergartners with PBEs. Our data provide a concrete set of metrics through which public health and education officials can identify high-risk areas as targets for policy and programmatic interventions.