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Children's Vaccines Health Center

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Mumps Vaccine Good, Not Perfect

2006 Mumps Outbreak Biggest in 20 Years Despite Vaccination
WebMD Health News
Reviewed by Louise Chang, MD

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.

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