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New Castle County estimates 15,200 cases of COVID-19 with sewage study

Courtesy of New Castle County government

There could be thousands more cases of COVID-19 in New Castle County than previously known — according to an unconfirmed study of the county’s wastewater.

New Castle County partnered with a Massachusetts-based company that uses sewage to estimate the incidence of COVID-19 in a given area. 

Biobot Analytics used genetic fragments of the SARS-CoV-2 virus in samples from the Wilmington wastewater treatment plant to estimate that roughly 15,200 people served by the plant were likely infected with the virus as of April 15. That’d be around 15 times the number of cases confirmed by tests in the county at that time. 

The company estimated an infection rate of 3 percent for the waterwater treatment center's catchment area, which is above the C&D Canal.

County Executive Matt Meyer says the Biobot study is an imprecise estimate, but a useful snapshot. 

“It’s very helpful as we try to determine where we are on the curve,” he said. “Where we are on the slope, have we flattened the curve, or is the curve decreasing.”

The County sent in another sample this week, and plans to do so again next week — so it can track the estimate over time. 


The partnership with Biobot was County Public Works Stormwater and Environmental Program Manager Mike Harris’ idea.

“The good thing about wastewater is it collects everything from everybody,” he said. “There's no privacy issues, there’s no issues with getting an incomplete sample like we have with our current testing situation, so it’s very broad-based.”


Some infections of the virus are asymptomatic. Gov. John Carney has said testing capacity needs to be ramped up in Delaware to get a better indication of the virus' spread before reopening.

“Having a high number, you would think on one hand it causes alarm,” said Meyer. “That means there are a lot of asymptomatic people who have the virus and are shedding virus. On the other hand it means we’re getting much closer to herd immunity than we ever thought.”

Meyer says he is not going to “make any grand decisions” or advise the Governor on when to reopen based on the Biobot estimate. 

The state Division of Public Health says it was not aware of the study, and cannot comment on it at this time.


Correction: A previous version of this story stated the estimate of 15,200 was extrapolated to include all county residents. The estimate is merely for the Wilmington wastewater treatment plant's catchment area.


Sophia Schmidt is a Delaware native. She comes to Delaware Public Media from NPR’s Weekend Edition in Washington, DC, where she produced arts, politics, science and culture interviews. She previously wrote about education and environment for The Berkshire Eagle in Pittsfield, MA. She graduated from Williams College, where she studied environmental policy and biology, and covered environmental events and local renewable energy for the college paper.