Exempted: Spatial Clustering of Intentionally Unvaccinated Children in California and its Potential Consequences for Measles Transmission

Rates of non-medical vaccine exemptions have been increasing in the U.S. since the early 1990s. These exemptions tend to be spatially clustered rather than randomly occurring in the population. In contrast to long-standing public health findings, under-vaccination due to non-medical exemptions tends to be found among children with educated, affluent, non-Hispanic white parents rather than those with relatively disadvantaged backgrounds. This dissertation uses data on Personal Beliefs Exemptions (PBEs) in schools in California from 1998-2014 and an empirically-calibrated large-scale simulation experiment to understand how spatial clusters of these exemptions form and their potential consequences for measles transmission. This dissertation emphasizes the importance of interaction in social networks in structuring both the local context surrounding parents’ vaccine decisions and the patterns of physical contact through which disease spreads. Major findings show that residential sorting of parents into neighborhoods and schools based on socio-demographic characteristics associated with school-level PBE rates provides only a partial explanation for broader patterns of clustering. Additional analyses provide evidence of spillover effects between PBE rates and socio-demographic characteristics, particularly percent non-Hispanic white children, on PBE rates in nearby schools.A large-scale simulation of measles transmission using a synthetic population of youth in California in 2014 is then used to examine the potential consequences of PBEs for disease spread. Schools serve as hotspots for measles transmission regardless of the spatial locations of PBEs, although clustering of these exemptions within households and charter schools increase the opportunities for infections. Surprisingly, spatial pockets of PBEs may provide slight protection against measles transmission when outbreak sizes are small and the disease is introduced randomly into the population. Yet, clustering of exemptions can reduce protective effects of herd immunity to large measles epidemics, even when overall vaccination coverage in the population is high.This dissertation concludes by drawing connections between findings in the empirical chapters and linking the results to the larger theoretical context of social interaction in networks. Practical implications for vaccine policy and efforts to control vaccine-preventable disease are discussed. Overall, this project seeks to illustrate how decisions perceived as personal by parents contribute to collective health outcomes