Mumps vaccine effectiveness of a 3rd dose of measles, mumps, rubella vaccine in school settings during a mumps outbreak -- Arkansas, 2016-2017

Objectives: The largest mumps outbreak in the United States since 2006 occurred in Arkansas during the 2016-17 school year. An additional dose (third dose) of measles-mumps-rubella vaccine (MMR3) was offered to school children. We evaluated the vaccine effectiveness (VE) of MMR3 compared with two doses of MMR for preventing mumps among school-aged children during the outbreak.Study design: A generalized linear mixed effects model was used to estimate the incremental vaccine effectiveness (VE) of a third dose of MMR compared with two doses of MMR for preventing mumps. Methods: We obtained school enrollment, immunization status and mumps case status from school registries, Arkansas's immunization registry, and Arkansas's mumps surveillance system, respectively. We included students who previously received 2 doses of MMR in schools with >= 1 mumps case after the MMR3 clinic. We used a generalized linear mixed model to estimate VE of MMR3 compared with two doses of MMR.Results: Sixteen schools with 9272 students were included in the analysis. Incremental VE of MMR3 versus a twodose MMR regimen was 52.7% (95% confidence interval [CI]: -3.6%-78.4%) overall and in 8 schools with high mumps transmission it was 64.0% (95% CI: 1.2%-86.9%). MMR3 VE was higher among middle compared with elementary school students (68.5% [95% CI: -30.2%-92.4%] vs 37.6% [95% CI: -62.5%-76.1%]); these differences were not statistically significant.Conclusion: Our findings suggest MMR3 provided additional protection from mumps compared with two MMR doses in elementary and middle school settings during a mumps outbreak.