Blame the vaccine or even the close quarters on college campuses.

That’s a couple of the reasons that mumps cases in the United States may have reached their highest rate in 10 years.

As of early December, approximately 4,300 cases of mumps have been reported, according to the Centers for Disease Control and Prevention (CDC). The last time the United States saw a spike in mumps was 2006, with more than 5,000 reported cases.

Mumps cases tend to fluctuate year to year, according to the CDC. It appears that two outbreaks make up the bulk of this year’s cases, Dr. Manisha Patel, a medical officer at the CDC told CNN.

Arkansas has had about 1,870 cases, and Iowa has had 683 cases.

Other cases seem to be clustered at universities and colleges around the country, in places including Indiana, Illinois, Massachusetts, and Oklahoma. The University of Missouri reported more than 200 cases right before students left for winter break.

Mumps is contagious and caused by a virus transferred through saliva via coughing or sneezing. So it’s not unusual for colleges to see outbreaks, Patel added. Close contact of students in dorms and other facilities on campus makes it easy for the disease to spread.

“The underlying theme of where outbreaks do occur are in congregate settings,” Patel told CNN.

Read more: Get the facts on mumps »

Vaccine doesn’t always work

According to the CDC, all of the people who’ve come down with mumps had been immunized against the disease.

That’s usually the case in most mumps outbreaks, said Dr. George Rutherford, professor of epidemiology and biostatistics at the University of California, San Francisco, and director of the Global Institute of Health.

“Basically the question is, ‘Why?’” he told Healthline. “We have a good vaccine and people are vaccinated twice against mumps.”

Rutherford said usually there are multiple factors at play.

The first is that the immunization rate of mumps is only 85 to 88 percent effective. That means 12 to 15 percent of people (usually children) don’t become inoculated. That’s why children receive another booster.

But even with the second booster, around 15 percent of that group will still not become inoculated. So some people remain susceptible to the disease.

Another contributing factor is that the vaccine may wear off, Rutherford added. By the time students grow up and head to college it’s been about 16 years since their mumps booster.

“It’s just not as effective over time,” he said.

Still others may be victims of storage error, according to Rutherford. Vaccinations that derive from live agents, such as mumps, must be stored at specific temperatures to remain potent.

The recommended storage temperature for the mumps vaccine is between -58 degF (-50 degC) and 46 degF (8 degC). Anything outside that range and the vaccine can “die” and won’t be effective when administered.

Read more: Get the facts on the MMR vaccine »

When mumps was common

Prior to the vaccine, the United States averaged nearly 200,000 cases of mumps each year.

The CDC says the illness typically starts with a few days of fever, headache, muscle aches, tiredness, and loss of appetite, followed by swollen salivary glands.

Symptoms tend to appear around 16 days after infection. Some people who get mumps have mild or no symptoms at all. Most people recover completely in a few weeks.

Complications from mumps include inflammation of the testicles in males who have reached puberty, inflammation of the brain, inflammation of the tissue covering the brain and spinal cord, inflammation of the ovaries, and deafness.

Children in the United States were first immunized against mumps in 1967. The shot is part of measles, mumps, rubella vaccine, commonly referred to as the MMR. The first round of this three-in-one-dose is administered to children aged 12 to 15 months. The second round comes between the ages of 4 and 6.

Of the MMR concoction, mumps has the lowest rate of efficacy. The first dose yields about a 78 percent effectiveness rate, according to the CDC. The second dose bumps that efficacy rate up to 88 percent.

Both measles and rubella carry an efficacy rate above 90 percent.

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An extra shot

Rutherford added that this mumps outbreak will likely cause many people to get a third booster.

At the University of Missouri, school administrators are encouraging students to receive a third shot when school resumes in January.

These types of one-off recommendations surrounding a mumps outbreak are typical, Rutherford added. However, he doesn’t anticipate the CDC taking an official stance on a third booster for teenagers.

“Currently there is no recommendation for a third dose before you go to college,” he said.

In the meantime, he said we can expect to see these kinds of outbreaks pop up every now and then.

“The vaccine isn’t as effective as measles,” Rutherford said, “so mumps still perks at a lower level.”