Unlike outbreaks of measles, which have taken place in populations with significant numbers of unvaccinated people, the mumps outbreaks have occurred in communities with high rates of immunization and people who often have received both recommended doses of the vaccine.

Currently, the Centers for Disease Control and Prevention recommends that children receive two doses of the MMR vaccine — for measles, mumps and rubella — with the first dose at 12 to 15 months of age and the second at 4 to 6 years of age. That schedule appears sufficient to control mumps in the general population, according to experts who spoke Wednesday at a meeting of the Advisory Committee on Immunization Practices.

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But that immunity wanes over time, and evidence presented at the meeting suggests that waning immunity is contributing to the recent increase in outbreaks.

The committee did not recommend an extra shot for everyone. It voted unanimously to recommend a third dose of mumps-containing vaccine only for people who are determined by public health officials to be at increased risk for mumps because of an outbreak.

The committee’s recommendations are usually adopted by the CDC, which sets the immunization schedule for children and adults.

Of the 150 outbreaks that affected more than 9,000 people across the country between January 2016 and June of this year, half occurred in university settings, Mona Marin, a viral diseases expert at the CDC, told the committee. Young adults were at the highest risk for mumps. Of the 78 percent of patients with known vaccination status, 70 percent had received two doses of the MMR vaccine.

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Health officials say a major factor contributing to outbreaks is a crowded environment. College campuses, where large numbers of students circulate in classes, in dormitories and on sports teams, are primed for spreading the virus.

The mumps component of the MMR vaccine is about 88 percent effective when a person gets two doses; one dose is about 78 percent effective. By comparison, two MMR doses are about 97 percent effective at preventing measles, and a single dose is about 97 percent effective at preventing rubella.

Officials said there was limited scientific evidence on the duration of protection from a third dose and whether it protected against mumps-related complications. Data presented at the meeting showed no serious adverse effects and relatively mild reactions to a third shot.

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Syracuse University, which is experiencing an outbreak of mumps, said this week that it would offer a third dose to students to help contain the outbreak on its campus, which began in August. As of Monday, there were 27 confirmed cases and 45 probable cases involving members of the campus community, officials said. All of the cases have occurred among students vaccinated against the disease.

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The mumps virus is found in saliva and respiratory droplets and is spread through coughing, sneezing, kissing or sharing drinks or utensils. Illness typically starts with fever, headache, muscle aches, fatigue and loss of appetite, followed for most people by swollen salivary glands. That swelling is what causes a sick person's puffy cheeks and tender jaw.

Complications from mumps infection can include encephalitis, meningitis, painful swelling of the testicles or the ovaries, pancreatitis and hearing loss. Women who become infected with mumps during the first three months of pregnancy are at risk of miscarriage.

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Arkansas, which had a large outbreak last year that started in a community of Marshall Islanders, used a third dose of the MMR vaccine in schools to control further transmission. Officials said the booster dose worked extremely well.