April 9, 2008 -- The 2006 eight-state U.S. mumps outbreak was the first ever caused by "two-dose vaccine failure," the CDC says.
Failure, however, seems too strong a word. The vaccine is known to be 80% to 90% effective after two doses -- not 100% effective. That means that one or two out of 10 people exposed to the mumps virus will be infected, although they may have less severe disease than unvaccinated people who are infected.
And without vaccination, the outbreak's 6,584 cases would have swollen to tens or hundreds of thousands, says Jane F. Seward, MB, MPH, deputy director of the CDC's viral diseases division.
"Our experience with this outbreak in the U.S. in 2006 reassures us how effective the vaccine is," Seward tells WebMD. "Two doses of mumps vaccine are highly effective but not completely effective. But without the high coverage rate, we would have seen a much larger outbreak."
Nevertheless, the 2006 outbreak was the worst in 20 years. Eight Midwestern states -- Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota, and Wisconsin -- had 85% of the cases. Most cases were among college students, even though most had received two doses of the mumps vaccine during childhood.
"We really suspect that the high transmission settings in colleges -- and some waning immunity -- contributed to the U.S. outbreak," Seward says. "Mumps virus got in somehow, possibly transported from the U.K. although we don't know that for sure. It got into a college in Iowa and didn't get recognized -- because people think mumps doesn't occur if you're vaccinated, but of course you can get it. So mumps established a hold there, picking off one or two at most out of 10 people exposed in these very, very high-contact settings."
The last time the U.S. saw a mumps resurgence was in the late 1980s. That outbreak led to the adoption of a second dose of mumps vaccine for school-age children. Canada routinely vaccinates children with only one dose of mumps vaccine; ongoing outbreaks there closely resemble the U.S. outbreaks of the 1980s.
Might the Canada outbreaks spread to the U.S.? Seward says there's no sign this is yet happening.
"Maine has had some cases this year, and we looked carefully for connections with Canada but have not noticed any -- nothing that has become established here," she says. "We expect we are probably bombarded with mumps from all over the world a lot of the time, because a lot of the world doesn't use mumps vaccine. Our surveillance isn't good enough to detect every mumps case that comes in, but we did detect some transport in 2006 from the U.K."
The U.K. epidemic in 2004 and 2005 was blamed on the relatively high number of people who refused to vaccinate their children with the measles-mumps-rubella (MMR) vaccine. The U.K. epidemic involved over 10,000 cases of mumps -- an infection rate 50 times higher than that seen in the U.S. during the 2006 outbreak, Seward says.
The two-dose vaccine seemed so effective that the U.S. set a goal of eliminating mumps by 2010. The current outbreak has upset that plan.
"This outbreak was unexpected. We were down to very, very low levels of mumps in the country, and we are still trying to understand why it occurred," Seward says. "The problem is some waning of immunity after two doses, but we right now we are just watching the epidemiology very carefully to consider policy changes that might be needed."
Would the CDC consider recommending a routine third dose of mumps vaccine?
"We see no need for a routine third dose. The cost benefit would not be there," Seward says. "But if we see again some outbreaks starting and continuing, we would at that point start a third dose in an outbreak setting to see if it limited the outbreak. But it would have to be a large outbreak on a lot of college campuses to show a benefit."
Seward and colleagues report their findings in the April 10 issue of The New England Journal of Medicine.