Spatial clustering of exemptions linked to measles outbreaks

(HealthDay)—Spatial clustering of exemptions to measles vaccination reduces protection from outbreaks, according to a study published online Dec. 6 in Pediatrics.

Ashley Gromis, Ph.D., and Ka-Yuet Liu, D.Phil., from the University of California in Los Angeles, modeled measles transmission in a hypothetical population of youth aged 0 to 17 years in California. Outbreak sizes were compared under the observed spatial clustering of exemptions in schools before and after the policy change eliminating nonmedical exemptions with counterfactual scenarios of no postpolicy change in medical exemptions, no clustering of exemptions, and lower levels of population immunization.

The researchers observed a significant reduction in both average and maximal outbreak sizes with the elimination of nonmedical 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 nonmedical exemptions; however, ultimately, this allowed outbreaks with thousands more infections compared with random distribution of exemptions. The large-scale outbreaks associated with exemption clusters were only reproduced with random distribution of exemptions when population vaccination was lowered by more than 6 percent.

"Vaccine programs and policies must recognize that population-wide protection against disease outbreaks depends not only on overall vaccination coverage but on locations of nonvaccination," the authors write.