The Emergence of Spatial Clustering in Medical Vaccine Exemptions Following California Senate Bill 277, 2015-2018.

Objectives. To understand how the elimination of nonmedical vaccine exemptions through California Senate Bill 277 (SB277) may have resulted in increased spatial clustering of medical exemptions.Methods. We used spatial scan statistics and negative binomial regression models to examine spatial clustering in medical vaccine exemptions in California kindergartens from 2015 to 2018.Results. Spatial clustering of medical exemptions across schools emerged following SB277. Clusters were located in similar geographic areas to previous clusters of nonmedical vaccine exemptions, suggesting a spatial association between high nonmedical exemption prevalence and increasing rates of medical exemptions. Regression results confirmed this positive association at the local level. The sociodemographic characteristics of the neighborhoods in which schools were located explained some, but not all, of the positive spatial associations between exemptions before and after SB277.Conclusions. Elimination of nonmedical vaccine exemptions via SB277 may have prompted some parents to instead seek medical exemptions to required school vaccines. The spatial association of these 2 types of exemptions has implications for maintaining pockets of low vaccine compliance and increased disease transmission.