Context All US states require proof of immunization for school entry. Exemptions are generally offered for medical, religious, or philosophical reasons, but the health consequences of claiming such exemptions are poorly documented. Objectives To quantify the risk of contracting measles among individuals claiming religious and/or philosophical exemptions from immunization (exemptors) compared with vaccinated persons, and to examine the risk that exemptors pose to the nonexempt population. Design, Setting, and Participants Population-based, retrospective cohort study of data from 1985 through 1992, collected by the Measles Surveillance System of the Centers for Disease Control and Prevention, as well as from annual state immunization program reports on prevalence of exemptors and vaccination coverage. The study group was restricted to individuals aged 5 to 19 years. To empirically determine and quantify community risk, a mathematical model was developed that examines the spread of measles through communities with varying proportions of exemptors and vaccinated children. Main Outcome Measures Relative risk of contracting measles for exemptors vs vaccinated individuals based on cohort study data. Community risk of contracting measles derived from a mathematical model. Results On average, exemptors were 35 times more likely to contract measles than were vaccinated persons (95% confidence interval, 34-37), Relative risk varied by age and year. Comparing the incidence among exemptors with that among vaccinated children and adolescents during the years 1985-1992 indicated that the 1989-1991 measles resurgence may have occurred 1 year earlier among exemptors. Mapping of exemptors by county in California indicated that exempt populations tended to be clustered in certain geographic regions. Depending on assumptions of the model about the degree of mixing between exemptors and nonexemptors, an increase or decrease in the number of exemptors would affect the incidence of measles in nonexempt populations. If the number of exemptors doubled, the incidence of measles infection in nonexempt individuals would increase by 5.5%, 18.6%, and 30.8%, respectively, for intergroup mixing ratios of 20%, 40%, and 60%. Conclusions These data suggest the need for systematic review of vaccine-preventable incidents to examine the effect of exemptors, increased surveillance of the number of exemptors and cases among them, and research to determine the reasons why individuals claim exemptions.
