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Harvard has a mumps outbreak. Here's why the virus can have a field day on campus.

Several universities across the the country have been battling outbreaks of mumps this year. The latest collegiate epicenter is Harvard, where more than 40 students have fallen ill.

But unlike the worrying recent outbreaks of measles, these students aren't getting sick because they didn't get vaccinated. In fact, all of the Harvard cases had all been immunized against the virus, according to the Boston Globe.

So what's going on here?

Normally, high vaccine coverage rates mean viruses like mumps can't circulate very easily. This is because of an idea in public health called "herd immunity": Vaccines work best when a certain (high) percentage of people in a population are immunized. Herd immunity keeps diseases from spreading through populations very easily, and keeps vulnerable groups who can't be vaccinated (such as very young babies or people with allergies to vaccines) protected.

But mumps, which attacks the body's glands — like the salivary glands in the neck, as well as the testicles, pancreas, and ovaries — is different.

Even though the vaccine has been extremely effective at the population level, cutting cases by 99 percent since it was introduced in the 1960s, it doesn't work so well in every context. And the ongoing Harvard outbreak is a reminder that college students are particularly vulnerable.

College is the ideal place for mumps cases to erupt

measles mumps rubella vaccine Sean Gallup/Staff

The measles mumps rubella vaccine. (Sean Gallup / Getty)

The mumps virus circulates via saliva or mucus, and universities are ideal hotspots for a couple of reasons.

First: Students are clustered together in close proximity — coughing, sharing utensils and beverages, kissing, and touching dirty surfaces.

"Saliva sharing is much more apt to occur at college, where people are sharing beer, living in a dorm," said Patricia Quinlisk, the medical director of the Iowa Department of Public Health, which has been battling several mumps outbreaks since 2006.

The college lifestyle not only increases the probability of exposure — it also increases the intensity of the transmission. "Rather than sitting in a cubicle away from guy whose coughing, a student might go to a party, kiss a couple of guys, one who has mumps, and she's now exposed to a million [of copies of the] virus," Quinlisk said.

The other reason mumps spreads readily on campuses, explained Saad Omer, a professor at Emory University's vaccine center, is that the vaccine happens to start to lose its power in some people right at the college age.

People get their first dose of mumps immunization (together with measles and rubella) around age 1, and a second dose a year later. By the time they enter college, that's nearly 20 years after they've been immunized. So, Omer said, "Waning immunity coincides with the specific age group when people are going to college."

And the vaccine isn't the most powerful to begin with. After two shots, estimates of the mumps' vaccine effectiveness have come in as low as 79 percent, meaning that a group of people who get the vaccine would have a 79 percent reduction in mumps cases compared to a similar group of people who didn't get it. That's better than the flu vaccine (currently 60 percent effective), but worse than, for example, measles immunization, which is around 99 percent effective with two doses.

This makes getting vaccines all the more important

Mumps is a serious virus. It can lead to painful swelling around the ears and jaw, as well as in the pelvis and testicles. The virus typically causes a fever, head and muscle aches, and tiredness — symptoms that can last for a couple of weeks.

Complications from the virus — including deafness, sterility, and encephalitis or meningitis (swelling of the brain) — can be devastating. But these severe reactions are much rarer today, precisely because of the vaccine.

"In the pre-vaccine era, mumps was probably the leading cause of deafness in places like Iowa, and the most probable cause of male sterility," Quinlisk noted. That's no longer the case.

Even if people pick up the virus, when they're vaccinated they don't typically experience such terrible side effects, and some have no symptoms at all. "[The complications now are] nowhere near as serious as the complications in the pre-vaccine era, and nowhere near same rate," Quinlisk added.

If more people are vaccinated, the chances the virus will break out anywhere are also lowered. The herd immunity threshold for a vaccine varies depending on the disease and strength of the shot. For mumps, researchers believe the threshold should hover around 92 percent to prevent outbreaks. In the US right now, herd immunity for mumps typically falls around 90 percent.

Even though college campuses have a higher herd immunity, since students need proof of vaccination to attend, there are always people who can opt out for religious or medical reasons. Coupled with the fact that the vaccine may be starting to wear off in some, college students are at a greater risk of not only getting the virus themselves — but spreading mumps elsewhere.

"[Herd immunity is] working in our communities," Quinlisk said, "which is why we're not seeing outbreaks in nursery schools, businesses, and schools. The problem comes in college, and herd immunity is still very important there. We're not seeing these rampant outbreaks there because of herd immunity."

Iowa's health department has been experimenting with giving students a third booster shot as they start college to see if that helps prevent outbreaks. The results of that experiment won't be out until next year, Quinlisk said. For now, unless a more effective vaccine is developed, the best protective measure everyone in a community can do is continue to get their shots and to isolate themselves if they are diagnosed.

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