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
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.