Current landscape of nonmedical vaccination exemptions in the United States: impact of policy changes.

State-level implementation of mandates, and related allowances for medical and non-medical exemptions, varies greatly across the United States. Non-medical exemption rates have increased over the last two decades, with an increased risk of disease outbreaks in clusters of children with non-medical exemptions due to differences in state laws. Recent efforts to address non-medical exemption rates range from incorporating additional administrative requirements for exemptions and disallowance of any non-medical exemptions. Continued monitoring is needed to evaluate the impact of these changes on exemption rates, to develop optimal childhood vaccination policy across the United States.