Context: The risk of vaccine-preventable diseases among children who have philosophical and religious exemptions from immunization has been understudied. Objectives: To evaluate whether personal exemption from immunization is associated with risk of measles and pertussis at individual and community levels. Design, Setting, and Participants: Population-based, retrospective cohort study using data collected on standardized forms regarding all reported measles and pertussis cases among children aged 3 to 18 years in Colorado during 1987-1998. Main Outcome Measures: Relative risk of measles and pertussis among exemptors and vaccinated children; association between incidence rates among vaccinated children and frequency of exemptors in Colorado counties; association between school outbreaks and frequency of exemptors in schools; and risk associated with exposure to an exemptor in measles outbreaks. Results: Exemptors were 22.2 times (95% confidence interval [CI], 15.9-31.1) more likely to acquire measles and 5.9 times (95% CI, 4.2-8.2) more likely to acquire pertussis than vaccinated children. After adjusting for confounders, the frequency of exemptors in a county was associated with the incidence rate of measles (relative risk [RR], 1.6; 95% CI, 1.0-2.4) and pertussis (RR, 1.9; 95% CI, 1.7-2.1) in vaccinated children. Schools with pertussis outbreaks had more exemptors (mean, 4.3 % of students) than schools without outbreaks (1.5% of students; P=.001). At least 11% of vaccinated children in measles outbreaks acquired infection through contact with an exemptor. Conclusions: The risk of measles and pertussis is elevated in personal exemptors. Public health personnel should recognize the potential effect of exemptors in outbreaks in their communities, and parents should be made aware of the risks involved in not vaccinating their children.
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