(CNN) The MMR vaccine, given in two doses to children, protects against the measles, mumps and rubella. Yet the United States and Europe have seen more than a few outbreaks of mumps, a contagious viral disease, in the past couple of years.

This resurgence may be due to waning protection bestowed by the mumps portion of the vaccine, according to a new study published in the journal Science Translational Medicine . A simple booster shot at age 18 could thwart future outbreaks, the Harvard researchers believe.

"The pattern we observed is a lot more consistent with what we would expect from a waning vaccine," said Joseph A. Lewnard, co-author of the study and a postdoctoral research fellow at Harvard's TH Chan School of Public Health. This is "a reassuring message" so far as it's a relatively simple problem to solve.

Searching for answers

Mumps, which is typically spread through saliva and mucus, causes fever, muscle aches, loss of appetite and the characteristic puffy cheeks and jaw from swollen salivary glands. In 1977, the US Centers for Disease Control and Prevention began to recommend two doses of the MMR vaccine for children.

For the new study, Lewnard and Yonatan Grad, assistant professor of immunology and infectious diseases at TH Chan, examined data from six separate mumps vaccine effectiveness studies conducted in the United States and Europe. Their goal was to understand the underlying reasons for a series of mumps outbreaks on college campuses since 2006.

These outbreaks are troubling for two reasons, Lewnard and Grad wrote.

First, up to 10% of mumps infections in teenagers or adults may cause severe complications, including deafness, meningitis and involvement of the testes or ovaries. Children who get sick with the mumps typically do not experience such severe symptoms.

Second, most recent mumps outbreaks have occurred among people who had received the recommended two vaccine doses.

The researchers wondered, is the vaccine less effective against emerging and circulating strains of the mumps virus? Or does the protection provided by the vaccine wane over time?

'Broad protection' against all strains

The new study showed the vaccine to be effective in providing "broad protection against all different strains of the mumps virus," Lewnard said. However, this protection did not last forever.

"We estimated that protection lasts on average 27 years," he said. "We estimate that about 25% of people will lose protection and be at risk for mumps in about eight years and that 50% will be at risk in about 19 years, and 75% of people will be at risk within 38 years. So the timing at which people lose protection can definitely vary individually."

In short, the mumps portion of the MMR induces a shorter-lasting immune system response than either the measles or the rubella components has shown in laboratory studies, Lewnard said.

Lewnard and Grad modeled options for when a booster shot should be administered to maximize protection and found that a booster dose around age 18 should be best, though it is unclear how long the additional protection might last.

"That's the age people are beginning to congregate," Lewnard said, whether on university campuses or in the military.

Outbreaks typically occur in congregate settings where there is crowding and close contact. In such places, saliva is transferred person to person due to coughing, sneezing, kissing and the sharing utensils, lipstick or cigarettes.

In fact, the military already has an existing policy of giving everyone an MMR dose when they come into basic training because, years ago, they had a similar problem with outbreaks, Lewnard said.

Just last year, the Advisory Committee on Immunization Practices -- a panel of medical and public health experts who meet three times a year to offer vaccination guidance to the CDC -- recommended the use of a third "booster" shot of the MMR vaccine (even for people who are up to date on the recommended doses) during mumps outbreaks.

"This is a recommendation that is on the books that people should be aware of in case they are, for instance, at a college when an outbreak is reported," Lewnard said.

Infectious disease specialist Dr. William Schaffner said the new research "reinforces what we're doing already," namely, recommending a booster shot in situations of outbreaks.

'Milder' illness with the shot

College health authorities, in collaboration with local authorities, have given that additional dose of MMR during some recent outbreaks. "When you get in early, you find out it's confined to a sorority or just a football team, and you can identify a subgroup of students and vaccinate them and stop the outbreak very early," said Schaffner, who was not involved in the new research. "It doesn't turn it off like a switch -- so it is not a be all and end all -- but it is a contribution."

Yet there's an "underappreciated" fact about the MMR vaccine: Although it is not perfect in preventing illness completely, it does a good job preventing many complications, Schaffner said. "So your illness is milder, and this has been a striking feature of the mumps outbreaks that have been studied."

Among the unanswered questions, he said, is how long protection would last if people were given a booster shot. "That's a large unknown, and that's why, at the moment, the (immunization committee) has not recommended a third dose for everybody in adolescence."

Still, mumps outbreaks have happened "all across the country" and have been "noteworthy," he said. Communities or colleges that have experienced outbreaks have devoted substantial resources to defining the extent, identifying who needs to be vaccinated, educating people and creating quarantines.

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"That's why it's become so much a matter of current discussion," Schaffner said, adding that the United States does a good job of vaccinating against the mumps. In fact, nearly 92% of infants receive their first shot of the MMR in the US , and compared with the pre-vaccine era, there has been a more than 99% decrease in mumps cases, according to the CDC.

In Europe, Israel and other places, he said, vaccination rates are not quite as high, and outbreaks of mumps still occur. Travelers may bring the mumps virus with them, so US officials have been working "diplomatically" to get other countries to be thorough with vaccinations, Schaffner said.

"The more all the countries of the world use the mumps vaccine effectively, the less mumps we'll all have -- even though the vaccine is not perfect," he said.