Epidemiology of a mumps outbreak in a highly vaccinated island population and use of a third dose of measles-mumps-rubella vaccine for outbreak control--Guam 2009 to 2010

Abstract

Background: Despite high 2-dose measles-mumps-rubella (MMR) vaccine coverage, a large mumps outbreak occurred on the US Territory of Guam during 2009 to 2010, primarily in school-aged children.

Methods: We implemented active surveillance in April 2010 during the outbreak peak and characterized the outbreak epidemiology. We administered third doses of MMR vaccine to eligible students aged 9-14 years in 7 schools with the highest attack rates (ARs) between May 18, 2010, and May 21, 2010. Baseline surveys, follow-up surveys and case-reports were used to determine mumps ARs. Adverse events postvaccination were monitored.

Results: Between December 1, 2009, and December 31, 2010, 505 mumps cases were reported. Self-reported Pohnpeians and Chuukese had the highest relative risks (54.7 and 19.7, respectively) and highest crowding indices (mean: 3.1 and 3.0 persons/bedroom, respectively). Among 287 (57%) school-aged case-patients, 270 (93%) had ≥2 MMR doses. A third MMR dose was administered to 1068 (33%) eligible students. Three-dose vaccinated students had an AR of 0.9/1000 compared with 2.4/1000 among students vaccinated with ≤2 doses >1 incubation period postintervention, but the difference was not significant (P = 0.67). No serious adverse events were reported.

Conclusions: This mumps outbreak occurred in a highly vaccinated population. The highest ARs occurred in ethnic minority populations with the highest household crowding indices. After the third dose MMR intervention in highly affected schools, 3-dose recipients had an AR 60% lower than students with ≤2 doses, but the difference was not statistically significant and the intervention occurred after the outbreak peaked. This outbreak may have persisted due to crowding at home and high student contact rates.

Figure 1:

Epidemiologic curve of reported mumps cases on Guam from December 1, 2009— December 31, 2010 ¹Source: Case Reports, Guam, through December 31, 2010 ² DPHSS, Department of Public Health and Social Services

Figure 2:

Flow sheet of students eligible for participation in the third dose MMR vaccine intervention and surveys, Guam 2010

Figure 3a:

Mumps attack rates (cases/1000) by age group by two-week period, Guam, December 1, 2009— December 31, 2010

Figure 3b:

Comparison of mumps attack rates (cases/1000) post-intervention among eligible students who received the third MMR vaccine dose compared with those who did not receive the third dose, Guam, December 1, 2009— December 31, 2010 * >1 incubation period post-intervention