The number of U.S. children entering schools without mandated vaccinations has increased over the past two decades, forming pockets of low immunization. This study examines mechanisms underlying spatial clusters of non-medical exemptions in California. Although the current wave of vaccine hesitancy began with the controversy around vaccine and autism, Liu finds that the locations of children with autism, as well as that of alternative medicine practices, are not associated with the broad spatial patterns of exemptions. The role of sorting into different neighborhoods according to maternal education is also limited. However, a series of tests show that the diffusion of vaccine safety concerns among non-Hispanic white parents is likely to have contributed to the spatial clustering. Self-selection into private and charter schools has a strong impact as well as potential spillover effects. Findings suggest that the concentration of exempted children in a few schools is partly an unintended consequence of the charter school movement. Network interventions may be useful to reduce clusters of low immunization.