Measles in the “post-elimination” United States: Threats and implications of vaccine refusal and domestic immunization policy in the third millennium

Abstract

Although the implementation of the highly efficacious measles vaccine in the 1960s allowed the United States to achieve elimination status by 2000, anti-vaccine sentiment has been growing in recent years. Vaccine refusal may play an important role in the size, frequency, and dynamics of modern measles outbreaks and is thought to have facilitated the rise in measles cases in 2014 and 2015 to levels that have not been seen in two decades. Refusal can be quantified from the frequency of unvaccinated schoolchildren whose parents have obtained exemptions to school immunization requirements. The types of exemptions permitted and the specifics of the obtainment processes are determined by state law. It has been previously shown that children with exemptions may have a higher risk for measles infection than children who are vaccinated. Here, I examine the implications of state legislation for vaccine refusal and measles incidence. Using negative binomial models, I compare exemption rates and measles incidence rates from 2001-2014 across six policy categories. I find that states with more lenient exemption policies have higher exemption rates and greater measles incidence. Vaccine refusal has also been found to cluster geographically within social structures, allowing for pools of susceptibility to gather. The 2014 measles outbreak among Amish persons in Ohio and the 2014-2015 California theme park outbreak were both partially related to the clustering of low vaccine coverage due to vaccine refusal and cultural practices. I apply a spatial epidemiological model to a similar community of refusers in the Twin Cities, Minnesota metropolitan region. I find a great potential for measles epidemics within this structure of low vaccine coverage. This finding highlights the potential impact of vaccine refusal in the U.S. and the need for enhanced MMR coverage on the local, state, and national levels.