In 2016, California eliminated non-medical exemptions for vaccine requirements upon children’s entry to school. As a result, overall rates of children with vaccine exemptions decreased, which was heralded as a substantial increase in new students receiving all required vaccines. But soon afterward, schools began to see an increase in medical exemptions to vaccines and a new study shows that this can contribute to increased risk of large measles outbreaks. The study, “Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak,” uses a simulation model to examine how different vaccine coverage scenarios before and after the policy change contributed to potential measles outbreak risks, published in the January 2022 Pediatrics (published online Dec. 6). The realistic simulation model based on hypothetical population representations showed that school-level clustering of vaccine exemptions before California’s elimination of non-medical exemptions was sufficient to threaten local herd immunity despite high overall immunization coverage. Increases in medical exemptions post-policy change, which often occurred in schools with higher rates of non-medical exemptions, reduced the effectiveness of the policy change substantially by contributing to larger potential measles outbreaks than would have been expected without these increases in medical exemptions. The results from this study contribute to a growing body of research demonstrating that spatial clustering of exemptions, even at high overall levels of population vaccination, can lead to underestimated risk of large measle outbreaks. The authors observe that large-scale measles outbreaks, including those with similar numbers of cases to outbreaks in New York City and New York State in 2019, were only observed when children with vaccine exemptions were spatially clustered within schools and communities. The results indicate that policies strengthening vaccine requirements may be less effective in preventing disease outbreaks if alternative forms of exemptions are concentrated in existing low-immunization areas.
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The American Academy of Pediatrics is an organization of 67,000 primary care pediatricians, pediatric medical subspecialists and pediatric surgical specialists dedicated to the health, safety and well-being of infants, children, adolescents and young adults.