Pennsylvania health officials have confirmed 13 cases of mumps at Ridley High School in Delaware County, the latest education institution in the Philadelphia region to experience an outbreak of the contagious virus.

But school officials say the total may be higher. Ridley Superintendent Lee Ann Wentzel said late Tuesday that the district has identified 15 presumptive cases at the high school. Those sickened include six staff members, a student teacher, and eight students, Wentzel said.

“Our expectation is we will have more, just because of the nature of kids in a school environment and the nature of this virus,” Wentzel said.

Wentzel said five students at the high school of more than 1,800 have “incomplete immunizations,” including due to health reasons.

Yet health officials say mumps outbreaks can happen even in communities with high vaccination rates.

“This is particularly common in close-contact settings, such as schools,” said Brittany Laufer, a spokesperson for the Pennsylvania Department of Health.

Laufer said officials were still investigating what caused the outbreak, and “it is possible we may never know how the first case became sick.”

More than 100 Temple University students were sickened with mumps earlier this year, prompting the school to open a clinic that vaccinated more than 2,000 people.

The U.S. Centers for Disease Control and Prevention recommends two doses of the measles, mumps, and rubella vaccine (MMR) for children, the first at 12 to 15 months old and the second at 4 to 6 years old. If a person receives both doses, the vaccine is 88% effective against mumps, according to the CDC.

The school district last week began providing a third dose of the vaccine to all faculty, Laufer said. The CDC recommends a third dose during a mumps outbreak.

Some research shows the vaccine can lose its effectiveness over time.

Laufer said anyone who has not received at least one dose of the MMR vaccine “needs to be excluded from school until the outbreak is determined to be over" by public health and school officials.

Mumps is typically spread through saliva, and its symptoms include swollen salivary glands and a headache. If a vaccinated person gets mumps, the illness will likely be less severe than in an unvaccinated person, Laufer said.

Wentzel said the district has increased efforts to keep the school sanitized to prevent the virus from spreading further, instructing custodial staff to make sure soap dispensers are stocked and table tops and door handles wiped down.

“There’s nothing that beats washing your hands with soap and water with a good scrub,” Wentzel said.