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BACKGROUND: In Oregon, there has been an ongoing increase in the number of schools reporting non-medical vaccine exemptions among students. In 2012, 84% of kindergartens in the Portland Metropolitan Area had at least one student with a non-medical exemption, compared to just 69% in 2009. Lower vaccination coverage can reduce herd immunity in schools, putting them at greater risk for outbreaks of vaccine-preventable diseases. Maintaining herd immunity confers protection even to unvaccinated children by reducing the frequency and ease of disease transmission within the population. To better understand the danger of increasing non-medical exemptions and maintaining herd immunity, we examined measles vaccination coverage among Portland Metropolitan Area schools. We hypothesized that schools with more non-medical exemptions for the MMR vaccine do not maintain sufficient levels of vaccine coverage for herd immunity against measles.
METHODS: We conducted a retrospective analysis of measles vaccination data for kindergartens (n=423) and child facilities (n= 730) in the Portland Metropolitan Area during the 2012 school year. Based on quartile distribution of coverage, schools were categorized by level of exemption, ranging from “no exemptions” to “very high.” The average up-to-date measles vaccination coverage in these categories was compared to the CDC herd immunity requirements for measles (83-94% vaccination coverage).
RESULTS: In 2012, both kindergartens and child facilities with “very high” levels of MMR-exempting students failed to meet the herd immunity threshold for measles. Kindergartens and child facilities with no reported or “low” rates of exemption exceeded herd immunity requirements while “medium” exemption schools exceeded the requirements by <1%. “High” exemption schools met, but did not exceed the requirements.
CONCLUSIONS: Despite a rise in the number of Oregon schools reporting non-medical vaccine exemptions, many schools still maintain sufficient vaccination coverage to ensure herd immunity to measles. However, a shared ideology among parents and administrators regarding vaccination may explain the clustering of unvaccinated students into specific schools. Such institutions that fail to maintain herd immunity against measles are particularly at risk for future outbreaks. Educating parents, school administrators, and health care providers not only about individual vaccination but also about the importance of herd immunity may help prevent the resurgence of measles and other deadly vaccine-preventable diseases.