A large measles outbreak in Washington State has prompted officials to declare a state of emergency. As of March 4, 2019, the Washington State Health Department reported 71 confirmed cases,1 with the majority of cases among individuals who were not vaccinated or who had an unverified vaccination status. The outbreak is centered in a geographic cluster of persistently high vaccine exemption rates among young children.2 This cluster has persisted despite the overall reduction in vaccine exemptions in the state of Washington, subsequent to the introduction of legally mandated health care practitioner counseling of parents seeking vaccine exemptions.2