Health consequences of religious and philosophical exemptions from immunization laws - Individual and societal risk of measles

Context All US states require proof of immunization for school entry. Exemp tions are generally offered for medical, religious, or philosophical reason s, but the health consequences of claiming such exemptions are poorly docum ented. Objectives To quantify the risk of contracting measles among individuals cl aiming religious and/or philosophical exemptions from immunization (exempto rs) compared with vaccinated persons, and to examine the risk that exemptor s pose to the nonexempt population. Design, Setting, and Participants Population-based, retrospective cohort st udy of data from 1985 through 1992, collected by the Measles Surveillance S ystem of the Centers for Disease Control and Prevention, as well as from an nual state immunization program reports on prevalence of exemptors and vacc ination coverage. The study group was restricted to individuals aged 5 to 1 9 years. To empirically determine and quantify community risk, a mathematic al model was developed that examines the spread of measles through communit ies 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 contr acting 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 ri sk varied by age and year. Comparing the incidence among exemptors with tha t among vaccinated children and adolescents during the years 1985-1992 indi cated that the 1989-1991 measles resurgence may have occurred 1 year earlie r among exemptors. Mapping of exemptors by county in California indicated t hat 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 exempt ors would affect the incidence of measles in nonexempt populations. If the number of exemptors doubled, the incidence of measles infection in nonexemp t individuals would increase by 5.5%, 18.6%, and 30.8%, respectively, for i ntergroup mixing ratios of 20%, 40%, and 60%. Conclusions These data suggest the need for systematic review of vaccine-pr eventable incidents to examine the effect of exemptors, increased surveilla nce of the number of exemptors and cases among them, and research to determ ine the reasons why individuals claim exemptions.