By Stephanie Stahl

RIDLEY, Pa. (CBS) — The mumps outbreak at Ridley High School is spreading. The Pennsylvania Department of Health says there are now eight confirmed cases. The outbreak has affected both students and staff at the high school, officials say.

Mumps easily spreads in close contact, even when people are vaccinated. With a series of outbreaks across the country, doctors now know the MMR vaccine can wear off.

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The mumps outbreak at Ridley High School in Delaware County appears to be spreading. State health officials say anyone exposed to mumps who has not received at least one dose of the MMR vaccine needs to be excluded from school until the outbreak is over.

A letter sent to parents on Tuesday said the exposure may have occurred in the last few weeks and warned parents to watch out for symptoms.

Symptoms include puffy cheeks and a swollen jaw caused by enlarged salivary glands. There can also be fever, headaches and muscle aches.

“The mumps has an incubation period from the time when you’re exposed until the time you develop symptoms — on average 16 to 18 days and can be as long as 25 days,” Philadelphia Health Commissioner Tom Farley said.

The Ridley School District says nearly all of the school’s 1,800 students are vaccinated.

While that significantly reduces the risk of getting mumps, studies have shown protection can decline.

Around the country, there has been an increase of mumps outbreaks recently among young, vaccinated adults.

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Research shows immunity after the mumps vaccination lasts an average of 27 years and a quarter of people lose their protection after only eight years.

Currently, children receive two doses of the MMR vaccine by the age of 6.

But in outbreaks — like the one that happened at Temple University in March, where more than 170 people got the mumps — booster doses are recommended.

“Seems like the mumps thing is spreading around and you know, it’s people need to be vaccinated,” Temple student Jacqui Ebbert said in March.

The Ridley School District has initiated the process of providing a third dose to all faculty.

Mumps is most contagious five days after the onset of symptoms.

Isolation is the recommended treatment once someone has mumps. There isn’t a treatment.

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Most people will have a complete recovery in a few weeks, but severe infections may cause complications.

Stephanie Stahl