Background: In 2009–2010, we investigated a mumps outbreak among a highly vaccinated Orthodox Jewish population in a village in Orange County, NY, to identify risk factors associated with mumps transmission among persons with 2 doses of mumps-containing vaccine.
Methods: Demographic and epidemiologic characteristics were collected on students in grades 6–12 in 3 schools. A mumps case was defined as a student, who self-reported parotitis, orchitis, jaw swelling and/or a mumps-related complication or whose mumps illness was reported to the Orange County Health Department during September 1, 2009, to January 18, 2010. Log-binomial regression analyses were conducted separately for boys and girls as they attended different schools and had different hours of study.
Results: Of the 2503 students with 2 documented doses of mumps-containing vaccine, 320 (13%) developed mumps. Risk of mumps increased with increasing number of mumps cases in the class [≥8 vs. ≤3 cases: boys aRR = 3.1; 95% confidence interval (CI): 2.0–5.0; girls aRR = 2.6; 95% CI: 1.6–4.1] and household (>1 vs. 0 cases: boys aRR = 4.3 95% CI: 3.7–5.6; girls aRR = 10.1 95% CI: 7.1–14.3). Age at first dose, time since last dose, time between first and second dose, school, class size, number of hours at school per week and household size were not significantly associated with having mumps.
Conclusions: Two doses of mumps-containing vaccine may not be as effective in outbreak settings with multiple, prolonged and intense exposure. Additional studies are required to understand why such mumps outbreaks occur and how they can be prevented in the future.