2. Children whose parents refuse vaccines require a personal belief exemption (PBE) for kindergarten entry in California.

3. States with easier exemption policies have higher rates of exemptions.

4. Parents who refuse vaccines have different attitudes and beliefs about vaccines than parents who adhere to the recommended schedule.

5. Vaccine refusers are observed to cluster geographically (within neighborhoods, zip codes) and socially (within schools, pediatric practices).

6. Clustering of exemptions is associated with higher incidence of vaccine-preventable childhood diseases.

8. 5,626 kindergarteners with one or more PBEs attend schools with PBE rates > 5%. Of these, more than one-third are in schools with PBE rates >=20%.

9. Personal belief exemptions are clustered geographically within counties and by school type.

10. PBE rates within California schools suggest epidemiological “hot spots” where disease outbreaks can occur.

12. Dependent variables : (1) number of kindergartners with one or more Personal Belief Exemptions (PBEs) ; (2) PBE rate (number of kindergartners with one or more PBEs/total kindergarten enrollment).

13. Analyses include cross-tabulations and negative binomial regressions. Figure 1: County-level Personal Belief Exemption Rates, California, 2008.