Background: New York City (NYC) experienced a large outbreak of mumps in 2009-2010.
Objectives: To assess risk factors for acquiring mumps in households and mumps vaccine effectiveness (VE).
Methods: Mumps cases reported to the NYC Department of Health and Mental Hygiene were investigated using standard methods. Additional information was collected on disease and vaccination status of household contacts. Follow-up phone calls were made to eligible households to identify additional possible mumps cases. For this study, a case of mumps was defined as one meeting the CSTE case definition or having clinically compatible illness reported by interviewee at follow-up. Generalized estimating equations were used to model age at time of index case onset, time from last Measles-Mumps-Rubella (MMR) vaccine dose, and household density as risk factors for mumps. VE was calculated using secondary household attack rates.
Results: 311 households with 2,176 residents were included in the study. The median age of residents was 13 years (range <1-85), and 462 (21.2%) household residents were mumps cases by the study definition. Among 7-18 year olds with known vaccination status (n=845), two-dose MMR coverage was 83.0%. A multivariate model demonstrated that adolescents and young adults 10-14 years (OR=3.4, CI=1.6-7.1) and 15-19 years (OR=2.7, CI=1.2-6.1) were at highest likelihood of mumps. Time from last MMR and household density were not significant risk factors. The overall 2-dose VE estimate for secondary contacts aged five and older was 79.4% (57.4-90.1).
Conclusions: The effectiveness of two doses of MMR vaccine to prevent mumps in our study was 79.4%, consistent with other published VE estimates. The less than 90% 2-dose MMR coverage in the study population combined with the 79% VE may have been contributing factors to this outbreak. Achieving high two-dose MMR vaccine coverage remains the best strategy to prevent mumps outbreaks and may have limited the scope of this outbreak.