Areas of increased school-entry vaccination exemptions have a pivotal role in epidemics of vaccine-preventable diseases in the United States. California eliminated nonmedical exemptions in 2016, raising overall vaccine coverage and rates of medical exemptions. The present study examined how spatial clustering of exemptions contributes to measles outbreak potential pre- and post-policy change.

It modelled measles transmission in an empirically calibrated hypothetical population of youth aged 0 to 17 years in California and compared outbreak sizes under the observed spatial clustering of exemptions in schools pre- and post-policy change with counterfactual designs of no post-policy change increase in medical exemptions, no clustering of exemptions, and lower population immunization levels.

The study observed-

• The elimination of nonmedical exemptions markedly reduces both average and maximal outbreak sizes. However, increases in medical exemptions cause more than twice as many infections, on average, than if medical exemptions were maintained at pre-policy change levels.
• Spatial clustering of nonmedical exemptions renders some initial protection against the random introduction of measles infections; however, it ultimately permits outbreaks with thousands more infections than the randomly distributed exemptions.
• The large-scale outbreaks produced by exemption clusters are not reproduced by randomly distributing the exemptions until lowering population vaccination by >6 percentage points.

This study shows that despite the high overall vaccination rate, the spatial clustering of exemptions in schools is sufficient to jeopardize local herd immunity and reduce protection from measles outbreaks. Policies boosting vaccine requirements may be less effective if alternative forms of exemptions (e.g., medical) are concentrated in existing low-immunization areas.

Gromis A, Liu K-Y. Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak. Pediatrics.2022; 149 (1): e2021050971. https://doi.org/10.1542/peds.2021-050971