Unvaccinated children who had a vaccine exemption were 35 times more likely to contract measles compared with vaccinated children. The epidemic curve of measles determined that the resurgence started a year earlier among children with exemptions compared with vaccinated children.

The investigators also used mathematical modeling to determine that the prevalence of exemptions in a geographic region was also associated with disease risk in the nonexempt population in that region, with high local aggregation of individuals with exemptions (ie, clustering) being associated with greater measles incidence.

A second study used Colorado measles cases from 1987 through 1998 and found that children with exemptions were 22 times more likely to contract measles than vaccinated children. The absolute and relative risk of disease was highest among individuals with exemptions aged 3 to 10 years. This study also found that the frequency of exemptions at the county level was associated with county measles incidence, though there was no association between schools with higher rates of exemptions and school-based measles outbreaks.

Exemptions were more likely to be in the index case and first generation (eg, individuals that acquired measles following exposure to the index case) (14.5%) compared with later generations (eg, those who acquired measles from individuals in the first generation or later rather than the index case) (7.1%). Among vaccinated children who contracted measles, 11% contracted it from an individual with an exemption.