College students are coming down with an illness most people think hasn't been a problem in the US for years: the mumps.

Three students at Butler University, two at Indiana University, and two at Kansas State University have been diagnosed with the mumps in the past few weeks. Most had been fully vaccinated against the disease, according to their universities.

Mumps cases in the US dropped dramatically after vaccination for the disease began in 1977. But the disease has been making a comeback in the past 10 years, ever since a historic 2006 outbreak mostly among students at Midwestern colleges:

I spoke with Dr. William Schaffner, an infectious disease specialist at Vanderbilt University, about why the mumps vaccine doesn't work so well, why new mumps cases are probably being imported from Europe, and what can be done to stop this.

Libby Nelson: Most, if not all, of the students getting mumps recently had been vaccinated as kids. Why did they get sick anyway?

William Schaffner: The initial immunization is part of three vaccines that are given together — measles, mumps, and rubella. Of the three elements in the vaccine, the mumps portion is the least effective.

Although immediate protection is excellent, its capacity to protect wanes over time, for example, 10 to 15 years. So what happens is when you get to high school and particularly college age, many people in the population have waning immunity, such that if they encounter someone who’s excreting a lot of mumps virus, they’re likely to get infected.

When we get out to 10 to 15 years after immunization, effectiveness is thought to be somewhere between 80 percent and 85 or 86 percent. So that leaves a proportion, and it’s a substantial proportion, of people who were once vaccinated newly susceptible to at least what we call modified disease, a disease of lesser severity.

LN: But they still have to run into someone excreting the virus. Where are these cases coming from?

WS: [Mumps] is gone from the United States, basically. So it has to be imported from areas of the world where we still have mumps.

Usually someone in Europe is exposed to mumps, comes to the US during the incubation period, develops mumps here, and spreads it through close contact. That’s often the case in colleges, where there’s a lot of international interaction, and many of our cases of mumps introductions have been traced back to Europe.

LN: Why are so many Europeans getting mumps?

WS: Why don’t the Europeans do a better job? We’re nattering at them at the present time. They also have measles outbreaks, and some of the importations of measles come from our cousins in Europe — developed countries that do not have that tradition of enforcing immunization requirements the way we do.

They don’t have "no shots, no school" laws, for example, that we tell them they need to institute.

In some ways, Europe now has more measles and more mumps than many parts of the developing world, where the [World Health Organization] and its various agencies and the Gates Foundation have been very much more effective in distributing these basic vaccines. Now we have Europe as a major continuing reservoir of these so-called childhood communicable diseases.

LN: Many hear that these vaccine-preventable diseases are coming back and they blame the anti-vaccination movement in the US. Is it at fault here?

WS: It plays a lesser role in this particular circumstance. There are going to be some people affected in these outbreaks of mumps who avoided vaccination, but that’s not the primary issue here. The primary issue is that the protective effect of the immunization wanes.

Remember, as you said, the substantial majority of people who are affected are people who have actually been vaccinated, and that’s because the protection afforded by the vaccine diminishes over time. By the time you get into college, if you look at the entire population, assuming the entire population has essentially been vaccinated, the vaccine protects about 80 to 85 percent. So that leaves roughly 15 to 20 percent of people who once were vaccinated who are now newly susceptible.

LN: How serious of a disease is mumps?

WS: You can get complications of mumps: encephalitis, inflammation of the brain; meningitis, inflammation of tissues covering the brain. You can get complications of orchitis, which is inflammation of the testicles in males and the ovaries in women. There’s also a sense that if you get this mumps/encephalitis/meningitis, you can be left with hearing loss afterward. None of which is nice.

So there are potentially noteworthy complications, although deaths are pretty rare.

[People who have been vaccinated and have had their effectiveness wear off] are likely to have a modified illness. … It’s not as severe. The men are less likely to get orchitis, that is the testicles involved, and the ladies are less likely as well.

LN: Is there anything that can be done to fix the vaccine wearing off? What about a booster shot?

WS: That's recommended in outbreak circumstances — another dose of vaccine for populations at risk. It’s not a perfect solution, because the mumps vaccine itself is not a perfect vaccine. So this revaccination provides a partial answer, but it usually is not good enough to abruptly end the outbreak.