Vaccination status of children exempted from school-entry immunization mandates

Background: The child immunization schedule is one of the most successful public health interventions of the past century. Despite this success, parental vaccine hesitancy is widespread and increasing. One manifestation of vaccine hesitancy is rising rates of nonmedical or “personal beliefs” exemptions (PBEs) from school-entry immunization mandates. The 2014-15 Disneyland measles outbreak and other recent outbreaks of vaccine-preventable diseases suggest a strong association between exemptions and increased outbreak risk; however, the strength of this association depends critically on the true vaccination status of children with exemptions, which has not been previously assessed. Objective: To estimate the true measles-mumps-rubella (MMR) vaccination status of children with PBEs. Methods: We use surveillance data collected by the California Department of Public Health in 2009 and nonparametric multiple imputation methods to replace missing values in the surveillance data. Results:  While surveillance data show MMR1 coverage of 36% and MMR2 coverage of 11% among children with PBEs in 2009, imputation scenarios point to much higher coverage (76-93% for MMR1 and 56-69% for MMR2), but still well below levels needed to maintain herd immunity against measles. Conclusions: These estimates can inform models of the actual disease outbreak risk associated with exemptions (such as during the 2015 Disneyland outbreak) and can identify regulatory responses and school policy changes that may be good candidates for interventions to reduce exemptions.