Mumps is on the rise in some parts of the U.S. And that has left some public-health professionals wondering whether the current two-dose vaccine works as well as it needs to—and whether a mumps booster (essentially a third dose of the measles, mumps and rubella vaccine—the MMR) later in life might be warranted for some people.

A study published this week in The New England Journal of Medicine (NEJM) suggests that a mumps booster may substantially reduce the likelihood of outbreaks. The research found that the extra dose reduced the risk of the contagious illness by some 78 percent.

"This is the first study to show that there was a significant effect of the third dose in reducing the risk of mumps in individuals in an outbreak setting, when compared to two doses," says the study's lead author, Cristina Cardemil, M.D., M.P.H., a medical epidemiologist in the division of viral diseases at the Centers for Disease Control.

Cases of mumps hit a 10-year high in 2016, with 6,366 reported to the CDC, and there have been 4,240 cases reported as of Aug. 12 of this year. According to the CDC, 200 to 2,000 cases a year is more typical.   

Here's what you need to know about mumps outbreaks, the new research, and who should consider getting a mumps booster.

Outbreaks Among the Immunized

A once common contagious illness, the mumps can bring temporary but painful swelling of the salivary glands in the jaw, and complications such as swelling of the testicles or ovaries, or in rare instances, inflammation of the brain.

Most people recover fully within a few weeks, but occasionally, the CDC says, it can lead to sterility in men or long-term cognitive and physical disabilities.

Immunization via two doses of the MMR vaccine confers about 88 percent protection against mumps. The CDC currently recommends that children get their first dose between 12 and 15 months of age and their second between the ages of 4 and 6.

But many of the recent outbreaks have taken place in communities where most people are up to date on immunizations. Though experts are unsure what’s causing the current spike in those areas, some are exploring whether the two-dose MMR provides enough protection to last through adulthood. Others theorize that the increase could be the result of a mumps strain that isn’t well-covered by the current vaccine.

What This Study Found

The NEJM study analyzed data from the University of Iowa, which had a significant outbreak of mumps—259 cases—during the 2015-2016 academic year.

Before that outbreak, 98 percent of the almost 20,500 students at the university had received at least the recommended two doses. During the outbreak, the university offered a third dose, and 4,783 students got it.  

Not only were the students who got the third MMR far less likely to come down with mumps during the outbreak, but those who’d had their second dose 13 years or more before were at significantly higher risk of contracting the illness. 

Changing Strains, Waning Immunity

So what does this mean? The long-held assumption that the MMR's protection is lifelong might not be correct, experts say.

According to William Schaffner, M.D., a professor of preventive medicine at Vanderbilt University in Tennessee (who wasn't involved in the NEJM study), the vaccine's effectiveness against the mumps may diminish after 10 to 15 years. As the study's authors note, "... waning immunity probably contributed to the propagation of the outbreak."

This study also found that the likelihood of contracting mumps increased incrementally over time, with the lowest risk in the first two to three years after people received the vaccine—and the highest risk 13 or more years out, Cardemil says.

Waning immunity may not be the only factor, though. Close living quarters, such as college dorms—where a large number of immunized young adults can have prolonged and intense exposure to the mumps virus—create an ideal environment for an outbreak, Schaffner says.

But it's unlikely that the MMR is failing to guard against newer mumps strains, according to Janell Routh, M.D., a medical epidemiologist at the CDC’s division of viral diseases. That's because the dominant subtype of the virus, which the MMR targets, hasn’t changed since the CDC started lab surveillance of mumps outbreaks in 2006.

Booster Basics

Like the University of Iowa, other U.S. colleges and communities that have experienced an increase in mumps have offered a booster to residents in an effort to curb outbreaks.

And if you live or work at a college or in a community hit by an outbreak, you may want to consider it, Schaffner says. "There’s no harm other than a sore arm, and you might incur some benefit," he explains. "It’s a perfectly reasonable step to take."

But most people, Schaffner adds, probably don't need one. As Manisha Patel, M.D., leader of the measles, mumps, and rubella team in the CDC's division of viral diseases and a study co-author notes: "This study was conducted in the setting of an outbreak. Generally, it's in outbreak settings, where there is a lot of virus in circulation, that waning immunity may be more of an issue than it is in the general population."

Right now, the CDC isn't changing its MMR recommendations. But the Mumps Work Group of the Advisory Committee on Immunization Practices (ACIP, which helps guide the CDC on vaccines), has reviewed this research and other research on the efficacy and safety of a third MMR dose. The group meets again in late October and then in February 2018.

What ACIP will do is unclear. "We'll have a better answer closer to the next meeting," Patel says. "But certainly this study will be very useful in what the next set of guidance or recommendations will be."

As we await the working group's conclusions, experts advise that you make sure you're up to date on your MMR. People who are fully vaccinated and contract the mumps anyway are less likely to develop severe complications, Routh says.

If you've never had the mumps and didn’t get the MMR as a child—or don’t recall getting it—you can get vaccinated now. The CDC recommends one dose of MMR for adults who haven’t had the vaccine, and two doses if you’re an adult at higher risk, such as a healthcare worker, an international traveler, or a college student.

If you’ve already had mumps—or were born before 1957—you’re considered immune; it's rare for mumps to recur.

In addition, take these steps to protect yourself and others from mumps:

Wash your hands often. Wet your hands with water, work soap into a lather, and scrub for at least 20 seconds. If the mumps virus is on your hands, this helps prevent it from spreading.

Don’t share. “That includes food, drink, lipstick, utensils—just don’t do it,” says Susan Even, M.D., executive director of the Student Health Center at the University of Missouri, which experienced a mumps outbreak last fall. Mumps spreads by saliva and respiratory droplets that can cling to a variety of surfaces.  

If you have the mumps, stay home. Stay there in isolation for the five or so days you have symptoms. (Note that some people have only mild or nonspecific symptoms such as body aches, fatigue, and fever.) Call your doctor if you experience a puffy jaw or swelling of any other part of your body. You may require medical treatment.