SPRINGDALE, Ark. — For a while, it seemed like the mumps outbreak engulfing this northwestern Arkansas city would never end.

“The doors would fly open at 9 o’clock in the morning and the lobby would be standing room only,” said Sandy Hainline Williams, an immunization nurse at a community clinic here.

At one point, the clinic would see 40 people with mumps every day. Hainline Williams said she and other nurses became so adept at mumps testing — swirling cotton swabs between the cheek and the jaw while massaging the salivary glands — they began calling themselves “swab queens.”

The epidemic here has slowed since its peak last autumn. But what happened in Springdale has ramifications far beyond the gently rolling hills of the northwest corner of this state. Large and lengthy mumps outbreaks have been reported in multiple places across the US over the past couple of years and, worryingly, often among people who have been vaccinated against the disease.

Last year marked the second-highest annual case count of mumps in more than a quarter-century. All but 13 states have reported mumps so far this year.

The reason for the resurgence is a mystery. But officials fear that it could undermine the public’s faith in immunizations, critical public health tools that are already under attack from people who believe vaccines are more dangerous than the diseases against which they protect.

Sandy Hainline Williams works as an immunization nurse at the Dr. Joseph H. Bates Outreach Clinic in Springdale. At one point, the clinic would see 40 people with mumps every day. Kenneth M. Ruggiano for STAT

In Arkansas, health departments sometimes resorted to giving extra doses of the mumps vaccine to try to build immunity in certain people — and some of them still got sick.

“This is an attack on vaccine confidence,” veteran vaccine researcher Dr. Stanley Plotkin told the National Vaccine Advisory Committee — a panel that advises the leadership of the Department of Health and Human Services on vaccines — during a February meeting.

Earlier this year, the Centers for Disease Control and Prevention set up a mumps working group. That panel, which consists of CDC and outside experts, will meet again in mid-April to look for answers as to why mumps cases have been on the rise and what can be done to reverse the trend.

While mumps isn’t generally severe, there can be serious complications. Men and boys can develop swelling in their testicles — called orchitis — which mostly resolves without incident but can, on rare occasions, lead to sterility. Hearing loss, meningitis, or encephalitis — swelling of the lining of the brain or the brain itself — also occurs on occasion, but rarely.

The most common symptoms are facial swelling, flu-like aches, and fever.

Still, the disease can be no minor inconvenience. The mumps virus has an extraordinarily long incubation period — the time from exposure to illness can take anywhere from 12 to 25 days. That long incubation time means mumps epidemics can drag out over months. Unvaccinated children are excluded from school and can’t return until 26 days have elapsed since the last case was spotted.

Northwest Arkansas has the highest rate of nonmedical vaccine exemptions — children whose parents opt not to get them vaccinated for philosophical reasons — in the state. When told their children would have to stay home for a minimum of nearly a month, some parents relented and allowed their children to take the vaccine. But others did not. Some of those students may end up missing the entire school year; state authorities say some parents are livid about that turn of events.

Mumps vaccine is bundled into a combination shot that also protects against measles and rubella. Children are supposed to receive two doses of the MMR vaccine, the first when they are between 12 and 15 months old and the second between the ages of 4 and 6.

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That vaccine was made with a strain of mumps (strain A) that circulated in years past but is no longer seen. Laboratory testing suggests the antibodies it generates also protect against the strain currently causing disease (strain G). Still, doubts linger.

“It’s not unreasonable to think that the vaccine maybe worked better against strains A and B and doesn’t work quite so well against G,” said Dr. Dirk Haselow, Arkansas’ state epidemiologist.

The mumps component of the MMR vaccine performs the poorest. While the measles vaccine protects 97 percent of kids who get two doses, the mumps vaccine protects only 88 percent of children who get both doses. And a single dose of the mumps vaccine — which was the recommended regimen until 1989 — only protects about 77 percent of the time.

In the Arkansas outbreak, the vast majority — nearly 92 percent — of the children ages 4 to 18 who contracted mumps had received two doses of the vaccine, Haselow said. Nearly 60 percent of the confirmed Arkansas cases have been children in that age group.

Even though the vaccine doesn’t appear to be working as well as it once did, there is a notable benefit over the pre-vaccine days. Serious side effects are rarely seen in vaccinated people who contract the mumps.

Haselow said with an outbreak as large as the recent one in Arkansas, the state could have expected about 500 cases of orchitis. “We’ve seen 18. And we would have seen nearly as many cases of meningitis or encephalitis, and we’ve seen zero,” he said.

The mumps virus is transmitted in saliva, and a number of factors affect how well it spreads. Among them is what Dr. Manisha Patel, a pediatrician who heads the CDC’s mumps team, describes as the force of infection — whether conditions favor viral spread.

“If I had mumps and I cough right into your face, that might be a different risk … than if I had mumps but was standing over there and you and I never interacted,” Patel said.

Certain kinds of behaviors put some groups at greater risk for mumps transmission. There have been outbreaks on hockey teams, for instance. Players sometimes share water bottles and have face-to-face shouting confrontations that contribute to the force of infection.

Likewise, universities often become the center of mumps outbreaks, owing to the frequency with which college students may share drinks and cigarettes, to say nothing of increased sexual contact.

In Arkansas, universities played only an incidental role in the outbreak. At least at the beginning, Springdale’s large community of Marshall Islanders were the people getting sick.

A pamphlet written in Marshallese details the services at a clinic in Springdale. Kenneth M. Ruggiano for STAT

No one is quite clear how many people from the roughly 1,000 South Pacific islands that make up the Republic of the Marshall Islands live in Springdale. Citizens of the former US protectorate don’t need visas to live and work in the United States, and in the past 20 years, growing numbers, drawn by plentiful low-skill jobs, have flocked to Northwest Arkansas.

Estimates of the number of Marshallese in the Springdale area range from 8,000 to 14,000. They live in large family groupings, with multiple generations and sometimes several related families sharing the same dwelling.

“Everything about their culture sets them up for mumps,” said Hainline Williams, the nurse coordinator for the Marshallese and Latino communities at the Dr. Joseph H. Bates Outreach Clinic. “They share their food. They drink [from the same glass] after each other.”

Even at the peak of the epidemic, isolating oneself to avoid transmitting the virus wasn’t really an option. Nor was staying away from church or the weddings, funerals, or birthday parties that are central to the tight-knit community. “It spread and it spread and it spread and it spread,” Hainline Williams said.

“It’s very unnatural for you to say: ‘Oh, I’m not going to come and take part in that.’ Because culturally, that’s not us,” explained Dr. Sheldon Riklon, a Marshallese doctor — one of only two in the United States — who practices in Springdale.

To try to quell the outbreak, a nurse accompanied by a Marshallese translator went door to door offering extra doses of vaccine. The clinic dispensed 7,600 doses of MMR vaccine during the outbreak.

But members of the community couldn’t understand why their relatives and neighbors were still getting sick even after the vaccine, said Ioanna Bing, a community research coordinator in the Center for Pacific Islander Health at the University of Arkansas for Medical Sciences.

Bing, who is Marshallese, said some members of the community thought they might be getting faulty product.

It seems clear the vaccine’s problems could be a double-edged sword — eroding the confidence of vaccine believers while shoring up the resolve of vaccine opponents.

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Plotkin, the veteran vaccine researcher who sounded an alarm at the National Vaccine Advisory Committee, declined to be interviewed for this article, explaining in an email that he’s on the CDC working group and is still considering the nature of the mumps problem and how it can be fixed.

But at the February meeting, he raised the possibility a new mumps vaccine may be needed, acknowledging that was “a frightening proposal.”

It would be frightening in part because it would be no simple matter. A new mumps vaccine would have to be developed and tested widely to see if it was actually more effective. The task could take years and cost tens of millions of dollars.