Mumps Outbreak Worsens in Washington State, With Nearly 300 Reported Cases Hundreds of students have been told to stay home to avoid getting sick.

 -- Cases of mumps in an ongoing outbreak in Washington state have continued to rise, with at least 298 likely or confirmed mumps cases, according to the state health department and the Spokane Regional Health District.

There are at least 166 likely cases in King County, where Seattle is located, and 94 cases in Spokane, where at least 300 students were told to stay home due to concerns they were not up to date on their vaccinations, the health departments in those counties said Thursday.

Approximately a two-thirds of those infected were vaccinated, but the mumps vaccine can wane over time. A full two doses of the vaccine provides approximately 88 percent protection, while a single dose can provide 78 percent protection, according to the U.S. Centers for Disease Control and Prevention. Ten cases were reported in King County in November, according to the county health department.

“Because some people do not get lasting protection from the vaccine and mumps spreads easily from person to person, outbreaks can still occur in vaccinated populations," Dr. Jeff Duchin, health officer for public health, Seattle and King County, said in a statement earlier this month. "But, if unvaccinated, many, many, more people would become ill.”

A community should be between 75 to 86 percent vaccinated in order to prevent an outbreak of mumps, according to UNICEF.

Mumps cases rose dramatically in the U.S. in 2016, with approximately 5,311 cases reported, according to the CDC. Eight states have reported an outbreak of 100 people or more. The virus famously can cause swelling of the salivary glands, causing a swollen jaw or face. Other symptoms include fever, headache, muscle aches and tiredness. In rare cases, it can cause swelling of the testes, ovaries, breasts or the brain.

The state department of health is investigating the ongoing outbreak and testing possible new cases in the hopes of stopping the outbreak, officials said.