Unlike outbreaks of measles and whooping cough, which have taken place in populations with significant numbers of unvaccinated people, the mumps outbreaks have been occurring in communities with high rates of immunization and residents who often have received both recommended doses of the vaccine.
Federal officials said Thursday that they are looking into whether mumps immunity decreases over time and whether there would be benefits to a third dose. State and local health authorities are particularly interested in that additional shot as a preventive measure, Mona Marin, a viral diseases expert at the Centers for Disease Control and Prevention, told the Advisory Committee on Immunization Practices.
“Although the disease has not been serious, the disruption and expense it has caused for local and state health officials has been significant,” said Kelly Moore, who directs the immunization program at the Tennessee Department of Health and is head of ACIP's work group on mumps.
Currently, the CDC 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.
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.
The country's mumps vaccination program began in 1967, a point when the virus represented a universal childhood disease and roughly 200,000 cases were reported annually. Even with the 2016 spike in outbreaks, there has been a 99 percent decline in mumps cases compared with the pre-vaccination era.
The advisory committee, established more than 50 years ago to provide expert advice to the CDC, meets regularly to discuss all aspects of vaccine safety for children and adults. It has formed a mumps work group to review issues related to the outbreaks, including the duration of immunity after two doses and evidence on benefits of a third dose.
Data on both of those points are limited, Marin told the committee. Waning immunity also does not fully explain a higher incidence of disease among 18- and 19-year-olds but not older students during some college campus outbreaks.
Recommendations are not scheduled to be presented to the panel until next February. But its members said Thursday that they are moving as quickly as possible.
“Two universities just reported cases to me in the last three weeks, so I feel the urgency,” said Moore, the mumps work group chair. “As soon as we have sufficient evidence, we will bring it to the committee post haste.”
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. Pregnant women who become infected with mumps during the first three months of pregnancy are at risk of miscarriage.
Health officials say a major factor contributing to outbreaks is a crowded environment. College campuses, where large numbers of students circulate in classes, dormitories and on sports teams, are primed for spreading the virus.
In Arkansas, the ongoing outbreak started in a community of Marshall Islanders in the state's northwest. The Marshallese live in crowded housing conditions, often with 15 to 20 people in a home, state epidemiologist Dirk Haselow said in an interview. From there, the outbreak quickly spread across the state to include broader communities and all age groups.
It is expected to wind down in the next two months, Haselow said. “At its height, we had 50 new cases a day. Now it's down to five or less cases a day.”
Arkansas has used a third dose of the MMR vaccine in schools to control further transmission. “This booster dose has worked exceptionally well,” he said.
Even with the large number of cases, the incidence of severe complications has been dramatically reduced, especially among vaccinated children and adults, he said. There have been only 17 cases of orchitis, or inflammation of the testicles, for example, instead of the 700 to 800 cases that would have likely occurred without vaccination.
“It's clear the vaccines make the disease milder than what would otherwise be seen,” Haselow said.