Potential effects of California's new vaccine exemption law on the prevalence and clustering of exemptions.

Exemptions from childhood immunizations required for school entry have continued to increase among California kindergartners, and exemptions show spatial clustering within the state. A 2014 change in California's school-entry vaccine exemption law requires parents filing for an exemption to submit signed documents from a health care provider. However, the evidence presented here suggests that the policy change will probably not be sufficient to reverse the growing trend in vaccine refusals.