(CNN) The United States is being hit with the most mumps cases in a decade, according to the Centers for Disease Control and Prevention. The latest tally as of November 5 is 2,879 cases in 45 states and the District of Columbia.

Mumps is a contagious disease caused by a virus spread through saliva and mucus.The CDC recommends that children get two doses of the MMR vaccine , which protects against measles, mumps and rubella, but it is not 100% effective.

To respond to this year's high numbers, the Advisory Committee on Immunization Practices -- a panel of medical and public health experts who meet three times a year to offer vaccination guidance for the United States -- is considering recommendation of a third dose of vaccine. It is unclear what age groups this would involve.

Communities and campuses

Before this year, the largest number of yearly mumps cases was tallied in 2006, when outbreaks in multiple states affected more than 6,500 people, primarily college students living on Midwest campuses.

Each year, the number of mumps cases fluctuates within a range of a couple hundred to a couple thousand cases, explained Dr. Manisha Patel, a medical officer at the CDC. This year, just two outbreaks make up the majority of cases, according to Patel. Arkansas has had about 1,870 cases and Iowa has had 683 cases, according to state health department data.

Iowa had a large outbreak in a university setting with additional cases in the community, Patel said, and the outbreak in Arkansas is community-based. Four other states -- Indiana, Illinois, Massachusetts and Oklahoma -- have also reported more than 100 cases this year, with several university campuses implicated in these outbreaks.

Most of the cases in Massachusetts have occurred at Harvard. Additional infections, which may or may not be related to that school, have occurred among students within the Boston-Cambridge area, according to Dr. Alfred DeMaria, medical director and state epidemiologist at the Massachusetts Department of Public Health.

The outbreak at Harvard started at the beginning of the year and slowed in the summer, with additional activity this fall, he explained. Public health authorities are still looking into possible connections.

"The underlying theme of where outbreaks do occur are in congregate settings," Patel said, noting that college campuses and other areas where there is crowding and close contact is "typical for mumps" because of the sharing of saliva: by coughing, sneezing, kissing and sharing utensils, lipstick or cigarettes.

Ann Garvey, a deputy state epidemiologist at the Iowa Department of Public Health, said the outbreak in her state is not a recent phenomenon.

"Last year, we had over 400 cases, and this year (PDF), we're on track to have more than that," she said.

Last year, the outbreak occurred on the University of Iowa campus, where the CDC recommended that students receive a third dose of the MMR vaccine.

"It seems to have quieted down now," Garvey said. "Whether that was just the right timing and school was going into breaks or whether it was the third dose of vaccine or a combination of both, we can't say for certain, but we have not seen a resurgence of cases on that campus this year."

"Studies that have tried to examine the effectiveness of a third dose of MMR vaccine to control mumps outbreaks were limited because vaccination occurred after the outbreak started to decline," explained Huong McLean, an associate research scientist at the Marshfield Clinic Research Foundation, which has partnered with the CDC on the issue.

Currently, Iowa is seeing cases in the community across all age ranges. More are occurring in the 6- to 17-year-old range this year, followed by the 18- to 25-year-old range, said Garvey. "Two counties are being impacted more than others. It just seems to be a community spread," she said.

Whether looking at the state or national picture, certain themes emerge.

Imperfect effectiveness

"It is important to understand that if you have a room of 100 people who have been vaccinated with the mumps vaccine and you expose them to mumps, you're still gonna get 12 people who will develop mumps," Patel said. "The MMR vaccine effectiveness is 88% with two doses."

The CDC recommends that children get their first dose at 12 through 15 months of age and the second dose at 4 through 6 years. In people who receive just one shot, the vaccine is less effective.

The alternative MMVR vaccine, which protects against measles, mumps, rubella and varicella (chickenpox), is licensed for use in children between the ages of 12 months and 12 years.

"Most of these outbreaks are occurring among vaccinated people," Patel said, adding that the CDC and other public health partners are trying to tease out what that means.

Before the US mumps vaccination program started in 1967, the CDC received reports of 186,000 cases each year, though the actual number of cases was probably much higher since most doctors considered the mumps a typical childhood disease. Since development of the vaccine, cases decreased by more than 99% in the United States.

Outbreaks in recent years, most notably one involving NHL players in 2014, have spurred debate over the need for a third dose of vaccine.

"The vaccine is still our number one preventive measure," DeMaria said, adding that one possible explanation for a mumps outbreak would be that the vaccine is failing to protect against the currently circulating viral strain.

"Genotype G is the virus that's been circulating pretty consistently in the US since 2006," Patel said. She said that studies show the current vaccine is effective against this strain and that the CDC is not concerned about changing strains.

Another possible explanation of the current outbreak would be that the vaccine's powers wane over time, noted DeMaria. Garvey also observed that college students are "further out" from getting their original shots, and this may be a reason why they are susceptible. In such cases, a third dose of the MMR vaccine might help prevent infection.

An ongoing investigation

Mumps typically begins with fever, headache, muscle aches, tiredness and loss of appetite lasting a few days. Most people will then see salivary glands swell, causing puffy cheeks and a swollen jaw. Although the incubation period ranges from 12 days to 25, symptoms last at least two and usually more than 10 days.

Mumps can occasionally cause complications, especially in adults. These include deafness and inflammation of the brain, ovaries, breast tissue or testicles.

Since it is caused by a virus, mumps will not respond to antibiotics. Doctors generally recommend bed rest and over-the-counter pain relievers.

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"We have some overall guidance that states can use on whether or not they should think about implementing a third dose" of the MMR vaccine, Patel said. She noted that each community is unique, and since the individual states understand affected populations best, the decision to use a third dose is theirs, absent an official recommendation.

Meanwhile, the CDC is collaborating with the states and other partners on ongoing studies and working with the Advisory Committee on Immunization Practices on the third dose recommendation. That committee will base any decision it makes on the most recent evidence regarding of vaccine safety and effectiveness, said Patel.

"A number of different things have to fall into place," she said. "A timeline is difficult to ascertain."