Days after Harvard researchers reported that the mumps vaccine appears to lose its power over time, the once-dreaded disease is making a comeback in the Philadelphia region.

Public health officials in Chester County, Montgomery County, and the state of Delaware are reporting outbreaks of the highly contagious, viral illness.

So far, Chester County has seen 19 known cases of the mumps since March 13, but reports are continuing, Jeanne E. Casner, county health director, said Wednesday.

Montgomery County has two confirmed cases and a third possible case, county spokeswoman Kaitlyn Foti said.

Meanwhile, Delaware state officials said Wednesday that they know of nine confirmed cases. The infected individuals in the First State are between the ages of 21 and 35, said a state agency spokeswoman. That tracks with the Harvard study, which found that immunizations given in childhood as part of the MMR vaccine may require a booster shot for mumps protection by the time a child is ready for college.

Most of the cases in the three outbreaks have been linked to a Feb. 10 social dance event, Baile Mejicano (Mexican Dance), which was at the Chase Center on the Riverfront in Wilmington. One of the Delaware patients was working at another Baile Mejicano event on March 3 at the Chase Center. Another Delaware patient's exposure is still under investigation.

Common symptoms of the mumps, which can appear after 12 to 25 days of exposure, include swollen salivary glands, fever, headache, or even swollen testicles. Rare but serious complications can include inflammation of the brain, deafness, or sterility. But generally, people who get the mumps as adults after receiving the immunization as children do not suffer such severe complications.

The United States has been experiencing a resurgence of the mumps over the last decade but particularly in recent years, with cases climbing into the several thousands in 2016 and 2017. Many of the outbreaks have been on college campuses, where people may be in close contact. The virus can be spread through saliva or mucus; by sharing glasses, utensils, or cigarettes; and holding hands or kissing. Last year, Pennsylvania State University's main campus experienced an outbreak.

Nationally, children are recommended to get the vaccine at around 12 months of age and before starting school. However, given the recent outbreaks, the Centers for Disease Control and Prevention earlier this year recommended adults at risk of infection, such as during an outbreak, get vaccinated.

The Harvard study put the blame on waning effectiveness of mumps vaccinations over time rather than a new strain of the virus. The researchers recommended routine re-vaccination at age 18 and possibly booster shots even later in life. Other experts think boosters should be given earlier in adolescence.

Health officials in Delaware and the affected Pennsylvania counties are urging people who think they may have been infected or have been in contact with infected individuals to get in touch with their doctors or public health offices.

"Anyone exposed to mumps should contact their health-care provider to determine whether they have contracted the illness and to assess if they and their family or close contacts need to receive vaccination against the mumps," Chester County's Casner said.