ABSTRACT [FROM JOURNAL]
Receipt of childhood vaccinations in the US has been declining, and outbreaks of preventable infectious diseases have become more common. In response, in 2014 California implemented a policy change for exemptions from mandatory vaccines for school enrollment. Data on fifteen successive cohorts of kindergarteners enrolled in public and private schools between school years 2001–02 and 2015–16 were analyzed for changes in vaccination trends. The results show an increase in the prevalence and clustering of vaccine exemptions from 2001–02 through 2013–14, followed by a modest decline after implementation of a policy mandating health care provider counseling for vaccine exemption. Clustering of vaccine exemptions increased over the study period and was less responsive to the policy change than were exemption rates overall. Nor did the policy change uniformly reduce the clustering of at-risk students across counties. Trends in the use of conditional admission showed strong school-level clustering and remained relatively stable. The policy change was effective at reducing exemption rates but did not uniformly reduce clustering of exemptions. The results suggest the need to evaluate the causes of local-area clustering and to adopt a statewide policy that addresses clustering of vaccine exemptions within schools and counties.