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Localized New Castle County sewer analysis continues to suggest higher viral activity than testing

Courtesy of New Castle County government

New Castle County continues to track an estimate of coronavirus activity there based on its sewer. The estimates have varied widely week to week, but County leadership still sees them as useful.

The virus is detectable in stool. So the Massachusetts-based startup Biobot Analytics uses samples from the county’s sewer to estimate the number of cases and infection rate. 

Biobot estimates more than 40,000 people in the northern New Castle County sewer system were shedding the virus in their stool last week, down from close to 60,000 the week before. 

According to last week’s sewage, the virus is most prevalent in Brandywine Hundred, where Biobot estimates nearly 30 percent of people could be infected, up from just around 1 percent the prior week.

Biobot does not offer a range of error on its estimates — and it’s unclear how precise they are. 

Biobot’s case estimates are an order of magnitude higher than those confirmed through individual testing, which has revealed just around 6,400 cases in the whole of New Castle Countysince the start of Delaware’s outbreak. 

The Division of Public Health has distanced itself from the Biobot studies. DPH spokesperson Jen Brestel says the state is not using them to inform decision making and is primarily using its own epidemiologic surveillance data.

But County Executive Matt Meyer sees the sewer estimates as an “advance look” at where testing numbers go. He says they’re helpful in decision making.


“Opening everything up at this point based on what we’re seeing from our sewers does not make sense,” he said. 

Meyer sees the sewer estimates as "another data point" to be looked at alongside testing data. He hopes the localized sewer estimates can help New Castle County residents make personal decisions, such as whether to go out to eat at a restaurant or see family. His administration has also used the sewer estimates in choosing where to locate testing sites. 

“We're doing ten sites a week—five days a week, two sites a day—across our county, and where we site those depends in part on what we’re getting from our sewer data,” he said. 

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