The Philadelphia Department of Public Health is warning residents of a measles cluster reported among unvaccinated residents. There are six locations where people may have been exposed to the infection since late December.

There are currently four confirmed cases and two more possible cases, the health department said Thursday. They are connected to a case that was confirmed in a patient at a Jefferson Health office at Ninth and Chestnut streets on Dec. 19.

Of the four people confirmed to have measles, two of them have been hospitalized and released, city officials said. According to the health department, the two possible cases under investigation are connected to a confirmed case where the infected person attended a day care, a violation of quarantine instructions.

The health department is working to stop the cluster from spreading by identifying people who may have been exposed, checking their vaccine status, warning them that they may have been exposed and issuing quarantine and exclusion orders where necessary.

People who were at the following locations at the following times may have been exposed to measles:

• Jefferson Health building, 33 S. 9th. St. — Exposures took place Dec. 19 between 2-5:30 p.m.
• Multicultural Education Station Day Care, 6919 Castor Ave. — Exposures took place Dec. 20 and 21
• Children’s Hospital of Philadelphia Emergency Room, 3401 Civic Center Blvd. — Exposures took place Dec. 28
• St. Christopher’s Hospital for Children Emergency Department, 160 E. Erie Ave. — Suspected exposures may have occurred overnight Dec. 30 to mid-afternoon Dec. 31
• St. Christopher’s Hospital for Children inpatient unit 5 North, 160 E. Erie Ave. — Suspected exposures may have occurred between Dec. 31 and Jan. 3
• Nazareth Hospital Emergency Room, 2601 Holme Ave. — Suspected exposures may have occurred Dec. 31 and Jan. 2

Anyone who believes they may have been exposed to measles should quarantine by staying home and away from others, city officials said. They added that those who have not received both doses of the MMR vaccine, which treats measles, mumps and rubella, should talk with their health care provider about receiving the necessary doses.

Typically occurring in childhood, measles is a highly contagious respiratory infection caused by a virus, which commonly results in symptoms — usually appearing about a week or two after being infected — including runny nose, puffy eyes, cough and a fever. Symptoms are often followed by a rash.

A measles infection can linger in the air and on surfaces for up to two hours after an infected person leaves an area. For some, the illness can develop into a more serious disease that leads to brain infection, pneumonia and even death.

Two doses of the MMR vaccine, which has been safely used for decades, are 97% effective at preventing measles, according to Philly’s health department. The city has high vaccination rates, with at least 93% of children fully vaccinated against measles by age 6. About 90% of unvaccinated people in the U.S. will get measles if they come in close contact with an infected person, and one in five unvaccinated people who get measles will be hospitalized.

“Unfortunately, we are seeing cases of measles that have spread to vulnerable individuals including young children due to people declining vaccination and also failing to adhere to quarantine recommendations,” Health Commissioner Cheryl Bettigole said in a release. “We are asking all city residents who may have been exposed to measles to do their part to ensure that no additional infants are harmed by this infection.”

Last July, a measles case was reported in Montgomery County after an unvaccinated child was confirmed to have contracted the virus.