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

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.