Other strategies to contain the outbreak on campus included education about cough hygiene (for instance, cough into your arm or a tissue), and working with student health services to make it possible for infectious students to isolate themselves, arranging for them to miss classes and retake tests so there was no pressure to break isolation, setting up food delivery services, and providing masks for those who had to use communal bathrooms.

Even if the immunity has dwindled, “we know that two doses of M.M.R. decreases your risk of serious complications,” Dr. Routh said. An important rationale for vaccinating against mumps was always to protect against orchitis, an inflammation of the testicles which used to be relatively common in post-pubertal males who came down with mumps, affecting 20 to 30 percent of them, and sometimes leaving them with loss of testicular tissue, decreased sperm counts and concerns about future fertility. The rate was much lower in vaccinated young adults.

Mumps virus can also more rarely cause inflammation of the ovaries, and even more rarely, though more dangerously, deafness and inflammation of the brain; again, these complications appear to be even rarer in those who have been vaccinated, but they may still occur. And even with 6,000 cases, it can be hard to estimate the frequency of a complication like lasting deafness, which used to occur in about one in every 20,000 cases.

Of course, waning immunity can be a problem after college as well. Dr. Alan Lucerna, the program director for combined emergency and internal medicine at Rowan School of Osteopathic Medicine in New Jersey, reported in February 2017 on a case of mumps in a fully immunized adult, in an article titled “Still a pain in the neck after all this time.”

“A good number of patients who come to the E.R. and are subsequently diagnosed with mumps are vaccinated and come in thinking it was just a lymph node swelling,” Dr. Lucerna said. Adults who develop swelling at the angle of the jaw together with flulike symptoms should think about mumps, he said. “We probably underdiagnose it.”

Should parents worry about sending their children off to college with their two childhood doses of mumps vaccine? “No, we do know that two doses of vaccine is protective,” Dr. Routh said. The current recommendation is that a third dose, while safe, is only warranted for people felt to be at high risk by public health workers, like the Iowa students in the setting of an outbreak. The two childhood doses of M.M.R. do protect most people against getting mumps, and they also, of course protect against measles, a much more serious disease, not to mention rubella.

“We’ve heard some people say this proves the vaccine’s not working, but the vaccine probably prevented a lot more students from getting ill,” Dr. Quinlisk said. Immunity does not wane in everyone, so many people who are exposed don’t get sick. “We certainly commend the University of Iowa for having a mandatory vaccine policy which kept it from being worse and prevented complications.”

Parents might want to keep in mind the informal survey done at the Iowa vaccine clinics where students waiting to get their third M.M.R.s were asked why they had come: Were they motivated by the posters on campus bulletin boards or by fear of complications?

“The No. 1 answer by far was: My mother called me and told me to,” Dr. Quinlisk said. “So the emails that went out to parents had a significant impact, and we have told other college campuses to make sure they don’t forget to keep the parents in the loop.”