Individual and community risks of measles and pertussis associated with personal exemptions to immunization

Context The risk of vaccine-preventable diseases among children who have ph ilosophical and religious exemptions from immunization has been understudie d, Objectives To evaluate whether personal exemption from immunization is asso ciated with risk of measles and pertussis at individual and community level s. Design, Setting, and Participants Population-based, retrospective cohort st udy using data collected on standardized forms regarding all reported measl es 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 exemptor s and vaccinated children; association between incidence rates among vaccin ated children and frequency of exemptors in Colorado counties; association between school outbreaks and frequency of exemptors in schools; and risk as sociated 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 confoun ders, the frequency of exemptors in a county was associated with the incide nce rate of measles (relative risk [RR], 1.6, 95% CI, 1.0-2.4) and pertussi s (RR, 1.9; 95 % CI, 1.7-2.1) in vaccinated children. Schools with pertussi s outbreaks had more exemptors (mean, 4.3% of students) than schools withou t 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 exemp tors. Public health personnel should recognize the potential effect of exem ptors in outbreaks in their communities, and parents should be made aware o f the risks involved in not vaccinating their children.