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TLANTA — With the U.S. facing a growing number of mumps outbreaks, an expert panel that advises the Centers for Disease Control and Prevention recommended Wednesday that people who are at risk during outbreaks should receive an additional, third dose of vaccine.

The Advisory Committee on Immunization Practices unanimously voted to approve a third dose of mumps-containing vaccine as a tool for outbreak control, despite acknowledging evidence to support the practice is limited.

The recommendation states that people who have had two doses of mumps vaccine but who are identified by public health officials as being at risk in a mumps outbreak should receive the third dose.

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Mumps-containing vaccine is either the measles-mumps-rubella vaccine or a vaccine that contains those three plus the chickenpox vaccine.Experts hope the recommendation will help address a problem that has bedeviled public health authorities recently.

After years of very low numbers of mumps cases — fewer than 1,000 cases nationally a year — the disease has made a resurgence of sorts in the past decade or so. There were nearly 7,000 cases nationally in 2006 and more than 6,000 last year. As of early October, there have been 4,667 cases in 47 states and the District of Columbia so far in 2017.

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It’s long been known that the mumps vaccine is less effective than those that protect against measles and rubella. Still, two doses of mumps vaccine — the first given at between 12 and 15 months of age and the second sometime between the ages of 4 and 6 years of age — is thought to protect about 88 percent of people vaccinated.

However, as mumps outbreaks have increased, public health officials have noticed that children and young adults — especially the latter — who received both recommended doses of vaccine were among those contracting the disease.

Mumps graph
CDC

In some cases, including in a large outbreak of mumps in northern Arkansas earlier this year, authorities have resorted to giving additional doses of the vaccine. But the evidence proving additional doses overcomes the problem of waning protection has been lacking.

Last month the New England Journal of Medicine published a study that delved into the issue. It was based on date from a 2015-2016 mumps outbreak at the University of Iowa.

The researchers found that the longer it had been since a person received a second dose of MMR vaccine, the more likely it was that he or she would contract the mumps. Students who had received their second shot 13 years or more before the outbreak were nine times more likely than more recently vaccinated students to be infected.

Given the recommended timing of MMR vaccination, teenagers and people in their early 20s could be vulnerable to contract mumps at a time when their lifestyles put them at higher risk, if they come in contact with the virus.

On Wednesday, a veteran vaccine expert — Dr. Stanley Plotkin, who is not a member of ACIP — suggested the committee should recommend a third dose of mumps vaccine for all students entering university. But the idea was dismissed.

“We did discuss this question. But … we did not feel that we had any evidence to support a decision about a routine [third] dose outside of an actual exposure situation where there was an immediate need for that vaccine,” said Dr. Kelly Moore, an ACIP member who chairs a work group that advises the panel on mumps vaccine. Moore is director of Tennessee’s immunization program.

The mumps virus spreads in saliva; so activities like kissing, sharing drinks and sharing cigarettes fuel its spread. Mumps outbreaks have been common on university campuses, but they have also occurred among athletes who play team sports. Several National Hockey League teams, for instance, have seen mumps spread along their benches.

The mumps virus causes symptoms that are common to lots of illnesses — fever, headache, muscle aches. But the virus also infects salivary glands, leading to pronounced swelling on one or both sides of the face. It can also cause a painful condition known as orchitis — swelling of the testicles — in males who contract the virus after puberty.

Most cases are mild, but there are serious complications in rare cases, including encephalitis (inflammation of the brain), deafness, and infertility. Severe cases are less common among people who have been vaccinated but still contract mumps.

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