Tracking the spread of coronavirus through sewage could be improved by examining solid rather than liquid human waste, researchers found, enhancing a rapidly developing technique for containing the deadly disease through public health measures.

Although there is much existing research into tracking COVID-19 through wastewater, most study so far has focused on liquid influent samples. Authors of a paper published Dec. 7 in Environmental Science & Technology suspected that they could obtain more sensitive measurements of the virus from settled solids if it was indeed concentrating there, as many viruses do.

Infected individuals shed RNA from SARS-COV-2, the virus that causes COVID-19, in their stool. Scientists can then isolate and identify the RNA from sewage in wastewater treatment facilities. For the latest study, researchers started collecting samples of both from the San Jose-Santa Clara Regional Wastewater Facility in March, when the virus first prompted lockdowns in the U.S.

“That’s pretty much what we find in this paper, that that holds up,” Marlene K. Wolfe, a postdoctoral scholar with Stanford University’s Woods Institute for the Environment and co-author of the study, said in an interview with The Academic Times. “There are higher concentrations in the solid than the liquid samples.”

What’s more, the data they gathered from the sewer samples tracked alongside local infection rates, with higher concentrations of viral RNA correlating with higher rates of COVID-19 as documented by the county. 

That means the technique could provide a measurement of infections that’s not affected by testing bias, Wolfe explained. While everyone uses the toilet, not everyone who develops COVID-19 will get clinically tested and be included in the official tally of cases, whether they choose not to, don’t have access to a test or don’t realize they’re infected.

This method could potentially identify new outbreaks sooner, saving crucial hospital space, Wolfe continued. It could also indicate whether public health measures such as restaurant closures are effectively stemming the spread of the disease, as well as track whether transmission is decreasing as schools reopen or vaccines are distributed.

“To be able to show that this works well, it’s a really sensitive method, and it works to show the thing that we’re actually interested in, which is how many cases there are in the community — I think that’s the contribution of this paper,” she said.

Data collection for the study ended in mid-July. Now that the researchers are confident in their methods, they’re working with several wastewater treatment plants in northern California, both to build on the numbers and explore ways to make the data as useful as possible, such as in the way it’s presented. They’re taking solid samples daily and processing them within 24 hours, returning the data to the plants and relevant local public health officials.

The seven-day average of confirmed U.S. cases peaked again Thursday at 213,165, an 18% increase from two weeks prior, according to The New York Times, as at least 238,189 new incidences and 3,293 more deaths were reported.

In what Wolfe called a “huge” development that “is going to pay off far into the future,” the Centers for Disease Control and Prevention is setting up a National Wastewater Surveillance System meant to complement other efforts. In the future, such a project could help monitor other pathogens shed in feces as well, she said, such as those related to the flu.

On the researcher side, several different efforts to coordinate efforts have sprung up, Wolfe said, noting that her team’s method is “by no means” the only valid one out there.

“Generally, something that could feel competitive feels like people are sharing knowledge and working together,” she said. “In the name of pandemic response and trying to do something about COVID, we’ve all been doing similar work lately. So it’s definitely been an interesting landscape to operate in.”

The study “SARS-CoV-2 RNA in Wastewater Settled Solids Is Associated with COVID-19 Cases in a Large Urban Sewershed,” published Dec. 7 in Environmental Science & Technology, was authored by Katherine E. Graham, Stanford University; Stephanie K. Loeb, Stanford University; Marlene K. Wolfe, Stanford University; David Catoe, SLAC National Accelerator Laboratory; Nasa Sinnott-Armstrong, Stanford University School of Medicine; Sooyeol Kim, Stanford University; Kevan M. Yamahara, Monterey Bay Aquarium Research Institute; Lauren M. Sassoubre, University of San Francisco; Lorelay M. Mendoza Grijalva, Stanford University; Laura Roldan-Hernandez, Stanford University; Kathryn Langenfeld, University of Michigan; Krista R. Wigginton, University of Michigan; and Alexandria B. Boehm, Stanford University.