On Aug. 14, 2020, in the U.S., despite high rates of coverage (>90%) at the national level for at least 1 dose of measles, mumps, and rubella (MMR) vaccine, the size and number of measles outbreaks has increased. Given this increase, it is imperative for public health officials and policy makers to understand the contribution of vaccine refusal to the changing epidemiology of measles.

In a previous review, JAMA reported that unvaccinated individuals made up the majority (56.8%) of cases in measles outbreaks and that more than two-thirds (70.6%) of unvaccinated individuals who were age-eligible for vaccination had a nonmedical exemption to vaccination.