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

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.

The importance of increasing primary vaccinations against COVID-19 in Europe

“…This creates clusters with low vaccination uptake and favors more frequent and larger outbreaks, making population-level epidemic control harder. This phenomenon has already been reported for flu 9 , measles 10–13 and pertussis 12,14 and explored in modeling studies 15–18 . Here, we quantified vaccination homophily for COVID-19 and found that it contributes to a sizeable share of the reproduction ratio - the average number of secondary cases that a case generates.…”

Spatial distribution and determinants of childhood vaccination refusal in the United States

“…Vaccine hesitancy behavior has been shown to be spatially clustered. Identifying this geographic clustering in vaccine hesitancy is important for controlling and eliminating vaccine-preventable diseases, as they can increase the risk of disease outbreaks and transmission (Omer et al, 2008;Atwell et al, 2013;Ernst and Jacobs, 2012;Truelove et al, 2019;Masters et al, 2020;Gromis and Liu, 2022). The spatial clustering patterns we identify will point to two alternative explanations for spatially dependent behavior: social influence or social selection (Alvarez-Zuzek et al, 2022).…”

Social clustering of unvaccinated children: measles-mumps-rubella vaccination coverage in schools in the Netherlands

“…There is also a more diffuse group of people who fear side-effects and perceive the diseases as mild [22], but there is not one school identity linked to that group. 4 Vaccination coverages by school are routinely collected in various countries, including Germany and the United States [17,18,[23][24][25], but reporting is frequently done by averaging school coverages locally, by county or district [24,26]. Whereas this is very helpful for signaling which regions are at risk of infection, an average or even a local average does not fully capture the variability between schools, which is essential to fully understand the risk of outbreaks [25].…”