Oh college—those halcyon days of young love, bong hits, and communicable diseases. You know, because packing students into dorms and dining halls and frat parties is a great way to facilitate social interaction and spread viruses. At Harvard, a recent mumps out break has gotten more 40 students sick just a few weeks before commencement.

News of an outbreak of mumps, which infects salivary glands, has drawn antivaxxers like moths to a swollen red flame. They’ve seized on the fact that students sickened in Harvard’s outbreak were all vaccinated—supposed evidence that vaccines are full of lies.

It’s true the mumps vaccine isn’t the most effective, but that’s established fact, not a scandal. The efficacy rate hovers around 78 percent for the first dose, and 88 percent for the second dose. (Mumps is one part of the three part MMR vaccine that also protects against measles and rubella.) And like all vaccines, its efficacy can wane over time.

That means doctors expect to see anywhere from 200 to 2,000 cases of mumps every year—down from 186,000 before the vaccine was implemented. So this particular outbreak? “This is not an unusual situation,” says Cristina Cardemil, a medical epidemiologist at the Centers for Disease Control and Prevention. Even in a college, where students are usually required by law to have two doses of the MMR vaccine.

Actually, mumps is more likely to infect campuses, because it spreads through saliva, either through kissing, sharing food and utensils, or droplets in the air from coughs and sneezes. Because of widespread immunization, mumps isn’t common. But once it gets into a population, it’s quite contagious. “It can take just one or two people to have the disease spread pretty easily,” says Cardemil. The first two cases at Harvard were detected in February. It’s impossible to know exactly how many people have been exposed, but the 40 cases so far are still just a tiny fraction of Harvard’s undergraduate population of 6,600.

No vaccine is 100 percent effective, and scientists don’t know exactly why it fails in some fraction of people. With mumps, the vaccine is made from a live but weakened virus—which is a little different from the actual circulating virus that gets people sick. That’s good, because you don’t want the vaccine to get people sick. But the downside is that the immune system might whiff it. Gregory Poland, who heads Mayo Clinic’s Vaccine Research Group, says his lab has actually tried to study what why the mumps vaccine sometimes fails and didn’t get funding. Mumps just doesn’t kill enough people to be a public health priority.

It'd be better to find more effective vaccines for infectious diseases—especially those with notoriously low efficacy, like the pertussis vaccine. But a few infections in students doesn't mean it's time to shut down vaccination requirements. Just imagine what out breaks would look like if only, say, half of all students got vaccinated. Undervaccination contributed to last year's measles outbreak in California. And Angola is currently experiencing a resurgence of yellow fever thanks to low vaccination rates.

With vaccination, outbreaks will still happen—but they'll remain small. The past couple of years alone, campuses at Ohio State, Fordham, University of Illinois have had their own mumps outbreaks. This year, it’s just Harvard’s turn.

Go ahead. Make a Harvard joke. I’ve heard them all.1

1The author attended Harvard, during which time there was maybe an outbreak of scabies and definitely an outbreak of swine flu. During commencement week, she got pink eye.