In Vaccine, Alison Buttenheim and colleagues assess how well rates of non-medical exemptions (NMEs) accurately reflect vaccine coverage rates in a population of school-aged children, and what this means for risk estimates of disease outbreaks. The authors looked specifically at the measles-mumps-rubella (MMR) vaccine in California. Since getting an exemption in California is fairly simple, the authors hypothesized that parents or schools might opt for an exemption rather than track down the necessary paperwork on a child’s immunization status. Thus exemption rates would not give the full picture on vaccine coverage rates, and schools’ own surveillance would be more accurate. In this surveillance kids with exemptions will be recorded as completely unvaccinated, partially vaccinated, or even fully vaccinated. Using this school-based survey data from the California Department of Health, the authors generate coverage estimates that account for the potential of partial vaccine coverage for students with NMEs. The imputation scenarios suggest that MMR coverage among exempted kindergarteners in California is 10–50% higher than exemption rates would imply, but still remain below herd immunity thresholds. Based on their findings, the authors conclude that prior analyses of the relative risk of measles associated with vaccine refusal underestimates that risk by an order of magnitude of 2–10 times.