Association for Public Policy Analysis & Management

Childhood vaccination plays a prominent role in minimizing the incidence of vaccine-preventable disease (VPD). Beyond protecting individual children from VPDs, vaccination confers a positive externality because it contains the spread of infection in the population and contributes to herd immunity. To ensure widespread coverage, states have enacted school immunization mandates whereby proof of vaccination is required to enroll children in school and preschool programs. School immunization mandates have been highly effective in achieving target vaccination rates in the U.S. However, all states allow for some form of exemption from vaccination.

Medical exemptions reserved for children with genuine contradictions to vaccination are permitted in every state, subject to certification by a licensed physician. Nonmedical exemptions, which allow individuals to refuse vaccination based on deeply held religious or personal beliefs, are permitted by a majority of states conditional on state-specific requirements. While offering nonmedical exemption preserves parents’ autonomy to make medical decisions for their children, misplaced fear over vaccine safety and side effects in recent years has given rise to anti-vaccination sentiment and increased utilization of nonmedical exemptions. In 2015, California experienced the most severe measles outbreak in over a decade attributable to vaccine hesitancy and availability of nonmedical exemptions. The state responded to the outbreak with SB 277, a law that eliminated nonmedical exemptions in California by August 2016.

In this study we exploit the repeal of nonmedical exemptions in California, relative to a group of control states, using a difference-in-differences framework to explore the impact of the policy change on immunization rates for 4 school-mandated vaccines, and rates of medical and nonmedical exemptions. Our unique dataset combines information on state exemption provisions with county-level exemption rates and vaccine-specific immunization rates. Controlling for an extensive set of covariates, incidence of VPDs, and county and year fixed effects, we find that the repeal caused a 2% to 4% increase in vaccination rates in California. We also find a significant 3% decline in nonmedical exemptions, accompanied by a 2% increase in medical exemptions in counties that previously had high rates of nonmedical exemptions.

Our findings indicate the repeal of nonmedical exemptions in California was only partially effective in improving vaccination coverage, and may have led parents to substitute between medical and nonmedical exemptions. Consequently, the availability and concentration of medical exemptions may continue to leave certain counties vulnerable to future outbreaks of disease. While few preliminary studies have identified changes in the composition of vaccinated and exempt children following the enactment of SB 277, these studies have relied on descriptive statistics and time series data from California to approximate this response. Our paper contributes to the literature by providing causal evidence of the impact of the repeal of nonmedical exemptions in California, relative to a control group of states that were unexposed to the policy change.