Four cases of mumps have been confirmed, an additional five illnesses are probable mumps and another five are being investigated in King County.

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Health officials say an outbreak of mumps is emerging in King County.

Four cases of the viral infection have been confirmed, all of which were vaccinated.

An additional five cases in the county are considered probable. Another five are being investigated.

The cases are predominantly in the Auburn area, said County Health Officer Dr. Jeff Duchin in a Wednesday news conference. Five are linked to one family, he said.

The nine confirmed and probable cases involve students in the Auburn School District, ranging from elementary to high schools.

No one has been hospitalized, Duchin said.

Symptoms of the contagious, viral disease include fever, headache and swelling of the cheeks and jowl.

“To reduce the risk of becoming ill, everyone should be sure they are fully vaccinated against mumps with the MMR vaccine,” Duchin said in a statement from Public Health — Seattle & King County.

The vaccine works on average in 88 percent of the population, Duchin said.

Continued exposure to infected family members increases the likelihood of transmission, he said. There is no evidence to suggest the vaccine was bad.

“It’s not unusual to have outbreaks in highly vaccinated populations,” he said.

Anyone with signs of the infection should see a health-care provider, Duchin urged.

Four probable or suspected cases of mumps were reported in King County last year. A larger outbreak swept through the area in 2007, with 53 cases.

“This is a blip for us,” Duchin said of the recent outbreak.

Local health officials reported last week that six states in the U.S. had reported over 100 cases of mumps this year — Arkansas, Iowa, Indiana, Illinois, Massachusetts and Oklahoma.

Most people recover from mumps in a few weeks. In rare cases, the disease can cause brain and spinal-cord inflammation and deafness.

An infected person can spread the virus through coughing or sneezing, and by hand contact.

Young children who have not been fully immunized have the highest risk of contracting the disease. Most people born before 1957, when mumps was widespread, probably were infected and have natural immunity.

Other than vaccination, preventive measures include hand-washing, disinfecting surfaces and toys and avoiding infected people.