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Testing sewage for COVID is providing valuable data, but Philly seeks to get faster results

The results of Philly's wastewater testing program to monitor COVID are public, but more needs to be done to maximize their usefulness.

Frozen wastewater samples from the omicron surge in winter 2022 stored at Temple University to analyze the presence of COVID-19 because the city didn't have a wastewater testing program in place at the time.
Frozen wastewater samples from the omicron surge in winter 2022 stored at Temple University to analyze the presence of COVID-19 because the city didn't have a wastewater testing program in place at the time.Read moresupplied

Philadelphia is using its sewage to monitor COVID-19, but the promise of that early warning system is hindered by the reality of slow turnaround times.

Testing wastewater for COVID can detect case increases days before they are identified through public reporting of PCR and antigen tests, especially since many cases are now unreported.

Early in the pandemic, Philadelphia was one of the first communities nationally to look for clues in its poop. After dropping the effort, it is now doing so again with funding from the Centers for Disease Control and Prevention.

The city just started releasing information last week from the program, which shows the technique can work. But the turnaround time is still slow: The most recent data publicly available are from Sept. 19.

The lag is because the Philadelphia Department of Public Health currently sends samples to Michigan State University for testing. Lengthy quality control tests and the time it takes to ship samples all contribute to delays of at least three or four days— and sometimes weeks — for reliable results.

Adding to frustrations, shipments to Michigan occasionally get lost in the mail, causing gaps in the data.

By next month, if hires and equipment purchases happen, the city hopes to get test results in a day and a half, using a newly opened health department lab that can conduct genetic sequencing on COVID samples.

One local expert in wastewater testing, Charles Haas, a professor of environmental engineering at Drexel University, said the technology can be valuable, but the health department also should make the data easy to understand and interpret.

» READ MORE: Philly health department’s new lab a key resource for tracking COVID

“The more real time they can make it, the better,” he said. “The big question comes down to: What are the actionable items that are going to be triggered when they see particular results?”

Renewed mandates to require masking or proof of vaccination aren’t likely, Philadelphia Health Commissioner Cheryl Bettigole said.

The department could issue safety recommendations during significant COVID surges, using the wastewater results along with other indicators of COVID’s impact, such as work absences and emergency-room volumes.

The wastewater testing could provide the earliest warning of a steep increase in COVID’s presence, which could prompt the health department to sound an alarm.

“If we start to see danger signs there and we let Philadelphians know, not everybody’s going to mask,” Bettigole said, “but a lot of people are going to start taking precautions again.”

» READ MORE: Philly’s wastewater testing for COVID is up and running, but city has yet to share data

Tracking COVID’s ongoing impact

Through much of the pandemic, the PCR or antigen tests available at hospitals and pharmacies provided a reliable gauge of COVID’s spread because they were reported to health officials. Now that the rise of home testing has made those programs less reliable as a measure of community spread, wastewater testing has the potential to fill an information void.

After months of fine-tuning, the health department began sharing the results of wastewater testing on the city’s online COVID dashboard last week. On two occasions since June, wastewater samples revealed increased levels of COVID a few days before those increases became apparent through PCR and antigen testing.

The data also bolstered the health department’s understanding that waves of BA.4 and BA.5 omicron subvariants this summer weren’t causing an alarming rise in infections or hospitalizations, said José Lojo, an epidemiologist at the department.

“It’s a complement to what we already have,” Lojo said.

The Philadelphia program samples from three wastewater processing sites, twice weekly, and will do so until the end of the year.

That isn’t enough to precisely identify where in the city the virus is most prevalent. Houston, by contrast, samples from dozens of sites, including wastewater treatment plants, pumping locations, and manholes, providing information on COVID’s presence in specific neighborhoods, health officials there have said.

Philadelphia health officials said sampling more sites would cost money the city doesn’t have. The wastewater testing program is largely funded by a CDC grant, though the city did not say how much the grant provides.

» READ MORE: The clues are in the poop: COVID-19 sewage testing is coming to Philly

The city invested in wastewater testing in part because of its potential beyond the current pandemic. The technology can be adapted to test for other infectious diseases, such as monkeypox and polio, but is not yet used in that way in Philly.

Testing for polio, in particular, is high on the list of priorities, Bettigole said. Most Philadelphians are vaccinated against the illness, but polio cases in New York this year have shown that small under-vaccinated populations can be vulnerable.

“It’s a low-likelihood event here,” she said. “It’s not a no-likelihood event.”