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A disease that appeared to be on its way to being eliminated has made a recent resurgence as mumps outbreaks have multiplied in recent years.

Cases of the viral disease have spiked, jumping to 6,366 cases in 2016 compared to 229 in 2012.

Researchers have been trying to understand what’s behind this return of a disease that had largely been brought under control by vaccinations.

Who is being affected by the outbreaks?

Mumps is a viral disease that can cause swelling of the salivary glands, fever, and loss of appetite. In severe cases, the disease may result in swelling of the testicles, ovaries or the brain. These severe symptoms can later impact a patient’s fertility.

The virus responsible for this disease is spread through saliva or mucus from an infected person, via coughing, sneezing, or talking.

People typically get the first dose of the measles, mumps, and rubella (MMR) vaccine as a 1-year-old, with the second dose administered between 2 and 6 years of age.

Many of the mumps outbreaks from last year happened at universities, where students often live in close quarters with each other.

In the first half of 2017, at least 42 likely cases of mumps were contracted at the University of Washington. At least 317 people came down with mumps in the 2015-16 school year at the University of Illinois at Urbana-Champaign during an outbreak there. And between August 2016 and July 2017, around 3,000 people got mumps in an outbreak in Arkansas.

In all of those outbreaks, public health officials reported that most or all of those affected had received the two-dose MMR vaccine.

“The fact that outbreaks have occurred in vaccinated populations suggest that something may be going wrong,” Joseph Lewnard, PhD, a postdoctoral research fellow at Harvard’s Chan School of Public Health and lead author of a new study, told Healthline. But whether it’s the vaccine or the number of doses of the vaccine that’s at fault has been, he said, “a source of uncertainty for some years.”

Why is mumps back?

Researchers think they have an answer.

In the study, which examined data on vaccine effectiveness, the researchers found immune protection against mumps wears off on average about 27 years after vaccination.

Past laboratory studies have looked into the immune responses to the vaccines but found they were providing good protection, Lewnard said. So it seemed unlikely that the answer would be to create a new vaccine.

For his study, Lewnard took an outside-the-lab approach, examining studies of the vaccine since it was rolled out in the 1960s.

He found the estimates of how long the vaccine was effective varied and that “almost all the variations in estimates of vaccine effectiveness could be chalked up to how long it had been since they received their last vaccine dose.”

Moreover, over those decades, different strains of the virus that causes mumps have circulated, he said, but researchers haven’t seen a long-term change in the vaccine’s ability to immunize against mumps, no matter how it has changed.

The researchers suggest looking into whether a third dose, at age 18, or even boosters throughout adulthood, might address this reemergence of mumps.

Some of this was already suggested by previous research. In response, an advisory board for the Centers for Disease Control and Prevention urged the agency in October to recommend that people at risk of mumps due to a nearby outbreak get a third dose of the MMR vaccine.

However, it stopped short of recommending everyone get a third dose.

“Although we have good evidence that a third dose can reduce an individual’s risk of infection during an outbreak, we do not have sufficient data to say with confidence how long this increased protection from a third dose will last,” said Dr. Kelly Moore, director of the Tennessee Department of Health’s immunization program and a member of the Advisory Committee on Immunization Practices.” There are simply some gaps in experts’ understanding of how mumps immunity works at this time.”

Lewnard isn’t saying everyone should get a third dose either. But he is saying we should look into it. “Our recommendation is that a third dose, at least in the short term, can be effective, and a routine third dose in the general population is worth investigating,” he said.

Who should think about a third dose of mumps vaccination?

The people likely most at risk of mumps are those entering college. They’re old enough that the effectiveness of their two-dose regimen may have started to wane, and they’re about to be in very close contact with a lot of people on a daily basis.

But just going to college doesn’t mean you’re going to be exposed to the mumps unless there’s an ongoing outbreak.

If there’s no outbreak, but you’re still concerned, could you ask your physician for a third dose of the vaccine? Probably, but the benefits would likely be minimal if any.

“If you were to request a third dose outside of an outbreak situation, your doctor could choose to administer it; this would probably be an out-of-pocket expense for an unknown benefit,” said Moore.

“I would suggest that doing so is not necessary unless you have been identified by public health as someone at risk of mumps due to an outbreak affecting your social group, school, or community.”

Moore points out that those who do get mumps after having had the two-dose vaccine typically experience “a mild, uncomplicated illness.”

Hospitalizations, she says, are “quite rare.”