Mumps, once a common disease of childhood, was on track to be eliminated in the United States by the year 2010, thanks to widespread use of two doses of the measles-mumps-rubella vaccine in early childhood. Then, inexplicably, the largest U.S. mumps epidemic in two decades occurred in 2006.
In a study reported in last week's New England Journal of Medicine, researchers from the Centers for Disease Control and Prevention and several Midwestern state health departments examined the 6,584 cases in the Midwestern outbreak. They found that the incidence was four times higher among people 18 to 24 years old than in all other age groups combined. This was despite the fact that 84% of the mumps patients in that age group (and 63% of the patients overall) had received the recommended two doses of mumps vaccine.
"Close-contact living conditions, like on college campuses, helped spread the disease," says Jane Seward, the CDC's deputy director of the division of viral diseases and an author of the study. But it also appears that, in some vaccinated people, there was a decrease in immunity over time.
Before the mumps vaccine was available, tens of thousands of cases were reported monthly, Seward says. In 1977, the CDC issued a recommendation that all children receive a dose of the MMR vaccine at 12 to 15 months of age, and the number of cases dropped dramatically. Then, during the late 1980s, outbreaks of measles, though much smaller than the pre-vaccination era, led to a new recommendation in 1989 calling for a second dose of the vaccine between ages 4 and 6. That was followed by historically low rates of mumps -- fewer than 100 cases a year -- until the unexpected surge in cases in 2006. Although it may be that, for some people, the vaccine weakens over time, Seward says the 2006 outbreak would have been much larger if so many people had not received vaccinations.
"We haven't had any more mumps outbreaks since," she says. "We're watching closely. If we see sustained transmission again, we would offer to work with college and state health departments to offer a third dose.
"Two doses of MMR vaccine are very effective but not 100% effective."