By Stephanie Stahl

PHILADELPHIA (CBS) — The mumps outbreak linked to a Wilmington concert is growing, as new cases were confirmed Thursday and local health officials are expanding the investigation.

Study: Mumps Outbreaks Linked To Waning Vaccine Protection

Two-thousand people attended the concert and health officials say they’re at risk for mumps, as well as people who’ve had close contact with them.

The mumps outbreak started here at the Chase Center in Wilmington at a dance festival on Feb. 10.

The state of Delaware now has 11 people with mumps, Chester County has 19 and two pending, Montgomery County has 2 confirmed cases and one pending.

The mumps virus causes flu like symptoms and puffy cheeks with a swollen jaw .

It’s transmitted in the air and through close personal contact

“It’s a very contagious virus, so that when it’s introduced into a community, if you have individuals who don’t have protection against it, it really spreads very rapidly,” explained Dr. Camille Sabella, a pediatric infectious disease expert.

But most people have had the MMR vaccine, which is supposed to prevent measles, mumps and rubella. Children get two doses by the age of six.

New research has shown immunity after vaccination lasts an average of 27 years and a quarter of people lose their protection after only eight years.

“We expect 50 percent of people to retain protection for 19 years after receiving a mumps containing vaccine. Many people will maintain protection far longer,” said Joseph Lewnard, a researcher at Harvard School of Public Health.

Recently, updated vaccine guidelines allow for a third dose of MMR, in the event of a mumps outbreak.

“Additional doses either in response to outbreaks or to prevent outbreaks might be warranted,” Lewnard said.

Local health officials are now advising anyone who attended the concert or lives with someone who did to get in touch with a doctor. They might need a new dose of vaccine.

Doctors say symptoms of mumps can take up to 25 days after exposure to show up.

It’s usually not dangerous but there can be complications.

There’s been a growing number of mumps outbreaks around the country, with infections on several college campuses as well as with NHL players.