Officials are investigating an outbreak of mumps at a Delaware County High School.
One student and three staff members have been infected with the virus at Ridley High School, Riddley School District Superintendent Lee Ann Wentzel said.
Because three of the people infected are staff members, officials are looking at the possibility that their vaccinations wore off over time, Wentzel said, adding that students are required to show proof of vaccination at the start of the school year.
The district asked students who are sick, and especially those running a fever, to stay home from school and have their parents contact a doctor.
Mumps can be easily spread among people in close quarters through kissing, the sharing of cups and fluids or competing in sports, according to the Centers for Disease Control and Prevention. Symptoms include fever, headache, muscle aches, tiredness and loss of appetite. Swelling in a person's salivary glands causes puffy cheeks, a signature feature of the disease.
It can take nearly two weeks for mumps symptoms to manifest in an infected person.
There is no cure for the virus, but the vaccine has an 88 percent chance of preventing a new infection, the CDC says.
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The vaccine's effectiveness can wane over time, however, meaning a booster shot may be necessary to keep up the body's defenses against the virus.