What our wastewater can tell us about COVID-19 rates

Each week, gallons of wastewater sewage arrive in Rachel Poretsky's lab at the University of Illinois-Chicago.

It's not part of some long-running collegiate prank; it's the latest tool being used by public health officials to better track and predict the spread of COVID-19 throughout the state.

Poretsky's team of researchers, in partnership with the university system's Discovery Partners Institute, receives wastewater samples from 65 water treatment sites in 49 Illinois counties at least twice a week from each location.

Wastewater is a delicate way of saying “poop.”

“When we first started, we were getting liter bottles sent to us,” said Poretsky, an associate professor of biological sciences. “Now, they come in 50-milliliter tubes.”

Researchers test the samples to determine the level of COVID-19 and compare the results to earlier samples from the same sites in an effort to monitor the growth, decline or stability of infection within the population.

Public health officials say the wastewater data, when combined with other infection data such as standard testing and hospitalizations, can help “inform that state's COVID-19 response,” according to the Illinois Department of Public Health website.

Local public health agencies also are analyzing the data collected.

“Wastewater surveillance is a relatively new addition to our process for monitoring COVID-19, and the science behind the field is still developing,” said Dr. Rachel Rubin, colead and senior medical officer at the Cook County Department of Public Health. “But wastewater surveillance has the potential to serve as an early indicator of increased disease transmission.”

Proponents argue the surveillance program is even more important now with the prevalence of at-home testing kits, because those results don't get reported anywhere. Wastewater samples also cover those in a community who may be infected but show no symptoms and go untested.

“Wastewater doesn't care about that, either. People are pooping no matter what,” Poretsky said.

University of Illinois-Chicago student Eva Durance prepares a sample of wastewater to be analyzed for COVID-19 while associate professor of biological sciences Rachel Poretsky monitors the extraction process. Courtesy of the University of Illinois-Chicago

The information gathered is transmitted to the U.S. Centers for Disease Control and Prevention, which is collecting similar data in many other states, as well. Results from across the country are updated regularly on the CDC's own wastewater surveillance website at

Extracted genetic materials from the virus also are sent to Argonne National Laboratory in Darien, where they are analyzed to identify any variants, IDPH officials said.

Unlike the CDC, IDPH has yet to incorporate results from the wastewater surveillance program into its COVID-19 data website.

Currently, the wastewater data available at the CDC is several days behind but shows growing levels of COVID-19 at most sites statewide.

“But we're still at some of the lowest points we've seen, and much lower than where we were in January and February,” Poretsky noted.

Wastewater is analyzed routinely for environmental safety practices, but an ongoing systematic surveillance program is a relatively new public health strategy.

“Since COVID, it's definitely become more mainstream,” Poretsky said.

Yet it's not a silver bullet, experts agree.

“It has the power to be very good early detection,” said Dr. Emily Landon, head of the University of Chicago's infectious disease prevention and control program. “The question is whether or not we will act on the results. To the best of my knowledge, the results are considered by state public health officials, but there isn't consensus about how to use the data to inform the public, what to say or how to incorporate the information into mask requirements.”

Compared with other states, Illinois has one of the most comprehensive wastewater surveillance programs in the country. Data is spotty in many other states, including larger ones such as Pennsylvania and California where only a few sites are reporting samples.

“There's a lot of data that's not making it to the CDC,” Poretsky said. “Reporting to the CDC can kind of be a pain because it has to be formatted very specifically. So there are states that are doing it, but it's not ending up on the data tracker.”

Illinois received more than $6.5 million in federal funding for its wastewater surveillance operation, IDPH officials said. And there is no cost for facilities to participate.

Poretsky said she gets regular requests from wastewater treatment plants throughout Illinois to join the state's surveillance program. She also noted funding from the CDC has been approved through 2025.

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