Delaware Public Media

Sewage study estimates New Castle County could have had 39,400 COVID-19 infections last week

May 1, 2020

New Castle County has received the result of a second test for traces of the coronavirus in its sewer system. It could point to a dramatic increase in infections. 

Last week New Castle County announced an unconfirmed estimate of the extent of COVID-19 infections there based on traces of the virus found in county sewage. The estimate by Massachusetts-based biotech startup Biobot Analytics of 15,200 cases as of April 15 dwarfed the number confirmed by lab tests at that time. 

The newest result puts the estimate of county-wide infections at 39,400, or around 7 percent of the county population, as of April 22. There were less than 1,400 lab-confirmed cases in the county then.

Biobot can estimate the infection rate in a given area based on the contents of its sewer — because traces of the virus are shed in stool, says startup co-founder Newsha Ghaeli. 

“A good way to think about it is those are the active cases in the community,” she said. “What we need more information on, for example, is how long after a COVID patient has recovered are they still shedding the virus in their stool.”

Ghaeli last week recommended tracking the sewage-based estimate over time.

The report received by New Castle County from Biobot
Credit Courtesy of Brian Cunningham

“What’s important is looking at that number as a qualitative trend,” she said. “So over time week to week, comparing that number with the week prior will help us really understand how the levels of the virus are increasing and decreasing.”

County Executive Matt Meyer emphasized last week that the Biobot test produces an “imprecise estimate,” but he said change in that estimate over time is a data point to consider.  

Lab-confirmed cases of the virus have grown more slowly in recent weeks in New Castle County than in Sussex, which has been declared a coronavirus “hot spot.”  

Biobot says its accuracy will improve as it analyzes more samples. The company says it is building a model to account for “person-to-person variability” in shedding of the virus in stool.

Biobot is working with over 170 facilities in 36 states on its COVID-19 project.