Spatial clustering of exemptions to measles vaccination reduces protection from outbreaks, according to a study published in Pediatrics. Ashley Gromis, PhD, and colleagues modeled measles transmission in a hypothetical population of youth aged 0-17 in California. Outbreak sizes were compared under the observed spatial clustering of exemptions in schools before and after the policy change eliminating non-medical exemptions with counterfactual scenarios of no post-policy change in medical exemptions, no clustering of exemptions, and lower levels of population immunization. The investigators observed a significant reduction in both average and maximal outbreak sizes with the elimination of non-medical exemptions; however, increases in medical exemptions resulted in more than twice as many infections relative to maintenance of medical exemptions at pre-policy change levels. Some initial protection against random introduction of measles infections was provided by spatial clustering of non-medical exemptions; however, ultimately, this allowed outbreaks with thousands more infections compared with random distribution of exemptions.