Third Dose MMR Intervention During a Mumps Outbreak In a Highly-Vaccinated Population – Guam 2009-2010

Background: Mumps, a viral illness characterized by fever and parotitis, may cause serious complications including meningitis, deafness, and orchitis. Two doses of measles-mumps-rubella (MMR) vaccine are ~90% effective against mumps; two-dose MMR vaccine coverage is 92.4% among U.S. kindergartners.  Between 2006 and 2010, three large mumps outbreaks occurred among two-dose vaccinated populations in the U.S., including an outbreak in Guam among school-aged children during 2009-2010. 

Objectives: To describe the epidemiologic characteristics of the Guam mumps outbreak and health department resource utilization and to evaluate the impact of a third MMR dose for outbreak control.

Methods: Active surveillance was instituted and cases were reported to the Guam health department. Age-specific attack rates were calculated using school enrollment data. Public health staff completed resource utilization surveys. Schools were eligible for the third dose intervention if they had >90% two-dose MMR vaccine coverage, ongoing mumps transmission, and a mumps attack rate >3/1000. Intervention and follow-up surveys were conducted in May and October 2010 among all students eligible for the third dose.

Results: From 12/2009-11/2010, 489 mumps cases were reported (median age: 12 years; range: 2 months-79 years). Among 279 children ages 6-18 years with mumps, 269 (96%) had received ≥2 MMR doses. The attack rate was highest (16.5/1000) among children ages 9-14 years in seven schools before the intervention. In May 2010, 1100 (30%) children in these schools received a third MMR dose. The attack rate declined by 93% (1.1/1000) post-vaccination.  Public health staff spent ~7494 hours controlling the outbreak.

Conclusions: The Guam outbreak occurred primarily in school-aged children despite high two-dose MMR coverage. This provided an opportunity to determine if a third MMR dose is useful in mitigating mumps outbreaks in congregate settings and may help inform national policy on administering a third dose in future mumps outbreaks among highly-vaccinated populations.