Frozen sewage slows down coronavirus surveillance

A lab technician tests a sample
City and University of Illinois researchers are analyzing sewage samples to measure coronavirus in Chicago's population. It's challenging to collect samples during extremely cold weather when wastewater freezes. Provided
A lab technician tests a sample
City and University of Illinois researchers are analyzing sewage samples to measure coronavirus in Chicago's population. It's challenging to collect samples during extremely cold weather when wastewater freezes. Provided

Frozen sewage slows down coronavirus surveillance

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A promising new science aimed at predicting COVID-19 outbreaks in high-risk communities is running into a challenge: Chicago winter.

Public health officials were unable to collect some wastewater samples as part of a coronavirus surveillance program because the sewage froze during extreme cold snaps this winter. Other samples showed low amounts of virus at a time when omicron was surging through Chicago, according to reports provided by the city in response to a public records request.

The freezing issue was discovered as the highly transmissible omicron variant was infecting Chicagoans at record levels in December and January. Health officials and scientists working to analyze wastewater for evidence of the virus say the issue hampers their research but add that they are working to resolve the problem.

“Obviously, this is a very big concern for us,” said Laura Clements, senior project manager for the University of Illinois-based research group Discovery Partners Institute that is working with the city and state.

Clements described the obstacle as temporary and said scientists are learning from the process. It’s easier to test localized community samples in warmer climates, such as Houston or San Diego where similar research is being done, she added.

The idea behind the science is to get ahead of community spread by spotting the very earliest signs of infections. The virus can be found in human waste well before symptoms show up in people. In addition to recent sampling of wastewater in seven communities around Chicago, plus O’Hare International Airport, a broader analysis has been ongoing since 2020 that examines large amounts of sewage data to help predict potential virus outbreaks.

Chicago’s surveillance detected an increase in COVID in mid-December as omicron was becoming the dominant form of the virus – but then measurements plunged at all seven community sites by the end of the month.

Chicago health officials said they are working “to ensure this is not an ongoing issue and to understand whether sub-freezing temperatures affect the wastewater data.”

Measurements dropped after overnight low temperatures consistently dipped below freezing, according to CDPH data and local weather records.

At a sampling site near the Roseland and West Pullman community areas, for example, high readings were detected Dec. 22 but plunged five days later.

From Dec. 29 through Jan. 19, the COVID readings showed a substantial range and one reading was listed as not available. The sewer samples showed no coherent trend and didn’t align with case counts reported throughout Chicago.

The other six community-based sampling sites – located in Austin/Montclare, Chatham, Chicago Lawn/Ashburn, Lakeview/Uptown, Lincoln Park/Near North Side and Norwood Park/Jefferson Park – all showed similar up-and-down patterns during the omicron surge.

Chicago health officials insisted that the frozen samples didn’t impact “trends overall” related to early detection and mitigation efforts to treat Chicagoans. But they admitted that the frigid temperatures are a barrier to the community-level analysis.

“There is no denying wastewater surveillance is a scientifically and logistically challenging system and we are working hard to establish this new capacity and excited by its potential,” public health officials said in a statement.

Also, sewage is just one form of COVID monitoring being done at the city level, they added.

“This did not impact [the health department’s] ability to detect or respond to the need around the increase in incidence in late 2021 or the introduction of the omicron variant,” the statement added.

Scientists at the Metropolitan Water Reclamation District, the Chicago area’s sewer management authority, have been tracking viral counts since the early days of the pandemic in March 2020.

MWRD is constantly analyzing raw sewage as it flows into three treatment plants, a process that’s not encumbered by the freezing temperatures, said Kaylyn Patterson, a water reclamation district microbiologist.

“Frozen temperatures make it definitely harder to take samples,” Patterson said. “We don’t have that issue.”

The U.S. Centers for Disease Control and Prevention plan to roll out a national database in the next several weeks, a spokesman said.

Brett Chase’s reporting on the environment and public health is made possible by a grant from The Chicago Community Trust.

Brett Chase reports on environment and public health for the Chicago Sun-Times. Matt Kiefer is WBEZ’s data editor. Follow them @brettchase and @matt_kiefer.