Ready for more mumps? The U.S. has had at least three outbreaks thus far in 2014, including ongoing problems in Ohio, where more than 200 cases have been identified, New York City with many cases as well, and New Jersey, with eight cases diagnosed just this week. All three of these outbreaks share the basics: college students who were vaccinated appropriately as kids but nevertheless come down with aches, fever, and sufficiently swollen salivary glands.

These outbreaks follow closely on the heels of similar much larger outbreaks in the Midwest in 2006, followed in 2009-10 by a whopper among Orthodox Jewish teenagers in New York and New Jersey. And who can forget last year’s New Jersey outbreak of a few dozen cases linked to patrons at D’Jais, a nightclub that considered itself “one of the Jersey shore’s most talked about venues.”

What’s the deal here? Mumps is supposed to be long-gone, like measles and German measles and all those other pediatric infections that actually never have gone away. The mumps problem, though, is altogether different from the usual tale of anti-vaccination: crunchy parents bent on protecting their children by making them sicker than shit by assuring they contract severe, preventable infections, making the mumps vaccine something of the let’s-not-talk-about-it problem child of vaccination campaigns.

Here’s the secret: given with measles and rubella (German measles) as the MMR shot twice a lifetime—at infancy and around kindergarten—the vaccine is only good, not great. Most studies place the efficacy at around 80%—far lower than that seen with measles and hepatitis B, but much better than the currently available vaccines to prevent influenza, which comes in at about 65% efficacy. And adding a third vaccination to kick it up a notch, though discussed, is not currently recommended outside of exceptional situations.

Vaccine protection doesn’t last long enough because vaccines always are victims of their own success. In the modern world, immunity is maintained two ways—either by an additional vaccination every now and then, or by exposure to a person with the actual disease. In the early days of any new vaccine, there are still lots of natural cases around, so a person vaccinated 5 or 10 years ago will likely encounter a person with the honest-to-god real infection of interest. This sort of “hello-oops-goodbye” exposure is quite beneficial. It’s not enough to clinically infect the vaccinated guy, but does serve to bump up his immunity another notch. So when there were both natural disease circulating AND vaccine, these periodic exposures made it possible to vaccinate little kids and figure they were protected for the rest of their lives.

But with mumps and, more recently, the chicken pox vaccine, removing all those exposures to true disease means that a person only has the immunity induced by the vaccine itself, with no additional exposures to natural infection. So the vaccine-induced immunity runs out of gas after 10 to 15 years, and kids in their teens become slightly susceptible.

But that’s only half the equation. To develop an true infection, not only do they need to have slightly lowered immunity, but they also need to be packed tightly in that airless, sock-reeking, can-I-borrow-your towel? latter-day version of the Army barracks—the college dorm. There, with that unique congregate housing mix of population density, hurried intimacy, and perverse pride in displaying the extremes of hygiene, just about any low-grade infection has a chance to be amplified by exposure and re-exposure and re-re-exposure within a few days or hours—more than enough to overwhelm any slightly wimped out vaccine-produced immunity. In scholastic terms, sustained close contact to mumps case + waning vaccine-induced immunity = transmission and a secondary case.

But even though the disease occurs among vaccinees, it typically is much less severe than that seem among those never vaccinated. For example, in the old pre-vaccine days, mumps caused non-bacterial meningitis and encephalitis often, accounting for more than a third of all such cases nationally. Plus for boys, inflamed testicles (orchitis) happened in a third or more, though sterility was uncommon. For women, some older experts associated pre-puberty mumps with subsequent infertility.

Since the vaccine has been around, these sorts of complications are unusual. In the New Jersey and New York outbreak among young orthodox Jews, the rate of orchitis was about 7%—significantly lower than that encountered among never-vaccinated persons in the same outbreak. Ditto for those who develop chicken pox despite the chicken pox vaccine—they too develop a mild, less poxed-out version of that disease.

In other words, despite the apparent vaccine failures, the breakthrough cases actually are a ringing endorsement for vaccination. The relative infrequency of failure (mumps used to cause tens of thousands of cases a year whereas now it is considered news when a college has a few dozen cases) shows dramatic benefit for the population in general, whereas for the poor guy who actually catches the disease despite vaccine, well, the infection is kinder and gentler and the recovery decidedly less arduous.

So the mumps news from New Jersey should be further evidence that America ought to swallow its pride, ignore its strident anti-vaccine rabble-rouser contingent and join up with mighty Croatia to require that vaccination become as certain as death and taxes.