Updated 10:25 a.m.

Volunteers with the University of Pennsylvania have begun the laborious process of calling anyone who’s been in contact with someone who is diagnosed with COVID-19. The practice, known as contact tracing, is considered key to reopening society, but is largely regarded as having its greatest impact when there are fewer cases to track.

Contact tracing takes a lot of time and a lot of labor. When a COVID-19 case is confirmed, a tracer reaches out to the patient to find out who he or she has been in contact with in the days leading up to the diagnosis.

“It’s a little spooky,” said David Nash, founding dean emeritus of Jefferson’s College of Population Health. “Who’s at home with you, where’d you go, what’d you have to eat, who’d you talk to — it’s a stranger calling you, asking some pretty intimate questions.”

Next, the investigator reaches out to those individuals to inform them they’ve been exposed — without revealing the identity of the person who exposed them — and directs them to quarantine for two weeks from the date of their exposure, so they don’t infect anyone else. That way, instead of everyone being forced to stay at home indefinitely, only those with known exposure would be, and for a shorter amount of time.

“We do think that that contact tracing is essential to be able to safely reopen society,” said Philadelphia Health Commissioner Thomas Farley. “We have to try to contain this infection so that we don’t have to live like this, where everybody is separated from everybody else.”

Because it’s such an involved process, contact tracing is optimal when there is a small number of cases of an infectious disease. Ideally, it would have been used as a containment strategy on the front end of the coronavirus outbreak, to identify cases or clusters of cases, notify those affected, and get them out of circulation to prevent them from infecting others. But because cases soared exponentially in Philadelphia, as it has in other places, the Public Health Department quickly lost the capacity to trace contacts.

Though it may be of some use now, Farley said a strategy like this would be most effective once new cases are down to about 50 per day.

On Wednesday, Philadelphia recorded 615 new cases.

Penn’s effort is starting on a very small scale. Carolyn Cannuscio, director of research for the Center for Public Health Initiatives at the Perelman School of Medicine, said so far her team has trained a few dozen volunteers, most of them medical or nursing students, or students in the schools of public health or social work.

The volunteers are working with the team responsible for delivering positive COVID-19 results to prioritize those patients within Penn’s medical system deemed “high exposure risk” because they live in congregate settings, such as nursing homes, or because they work in high contact occupations. That group comprises the dermatology team, which was redeployed to deliver test results since their normal appointments and procedures have been largely canceled.

The team at Penn is in contact with the city Health Department, to let it know when they begin reaching out to the people these patients have been in contact with.

“The processes are still being hammered out,” said Cannuscio, noting that the volunteers made their first set of calls over the weekend. “It’s a more manageable process when there are fewer cases.”