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Gromis A, Liu KY. Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak. Pediatrics. 2022;149(1)doi: 10.1542/peds.2021-050971.
Gromis, A., & Liu, K. Y. (2022). Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak. Pediatrics, 149(1), . https://doi.org/10.1542/peds.2021-050971
Gromis, Ashley, and Liu, Ka-Yuet. "Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak." Pediatrics vol. 149,1 (2022). doi: https://doi.org/10.1542/peds.2021-050971
Gromis A, Liu KY. Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak. Pediatrics. 2022 Jan 01;149(1). doi: 10.1542/peds.2021-050971. PMID: 34866158.
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Pediatrics. 2022 Jan 01;149(1). doi: 10.1542/peds.2021-050971.

Spatial Clustering of Vaccine Exemptions on the Risk of a Measles Outbreak.

Pediatrics

Ashley Gromis, Ka-Yuet Liu

Affiliations

  1. Departments of Health Policy and Management.
  2. Sociology.
  3. California Center for Population Research, University of California, Los Angeles, California.
  4. Center for Brain Science, Riken Institute, Wako, Japan.

PMID: 34866158 DOI: 10.1542/peds.2021-050971

Abstract

OBJECTIVES: Areas of increased school-entry vaccination exemptions play a key role in epidemics of vaccine-preventable diseases in the United States. California eliminated nonmedical exemptions in 2016, which increased overall vaccine coverage but also rates of medical exemptions. We examine how spatial clustering of exemptions contributed to measles outbreak potential pre- and postpolicy change.

METHODS: We modeled 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 postpolicy change with counterfactual scenarios of no postpolicy change increase in medical exemptions, no clustering of exemptions, and lower population immunization levels.

RESULTS: The elimination of nonmedical exemptions significantly reduced both average and maximal outbreak sizes, although increases in medical exemptions resulted in more than twice as many infections, on average, than if medical exemptions were maintained at prepolicy change levels. Spatial clustering of nonmedical exemptions provided some initial protection against random introduction of measles infections; however, it ultimately allowed outbreaks with thousands more infections than when exemptions were randomly distributed. The large-scale outbreaks produced by exemption clusters could not be reproduced when exemptions were distributed randomly until population vaccination was lowered by >6 percentage points.

CONCLUSIONS: Despite the high overall vaccinate rate, the spatial clustering of exemptions in schools was sufficient to threaten local herd immunity and reduce protection from measles outbreaks. Policies strengthening vaccine requirements may be less effective if alternative forms of exemptions (eg, medical) are concentrated in existing low-immunization areas.

Copyright © 2022 by the American Academy of Pediatrics.

Conflict of interest statement

FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.

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