New Castle County government is dipping into local sewers for new estimates of how prevalent COVID-19 is in Delaware's most populous county.

Last week, county officials sent a sample of raw sewage from the Wilmington Wastewater Treatment plant on 12th Street to a Massachusetts Institute of Technology startup. There, they compared the prevalence of genetic fragments from the virus in fecal matter against local population data and sewage flow rates – all in an effort to estimate how many people may have the virus.

The company's analysis estimated that 15,200 people – 3 percent of the population north of the Chesapeake & Delaware Canal – had the virus as of April 14. That number is approximately 15 times the laboratory-confirmed cases in the county as of the sample date.

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As of Friday, New Castle County had 1,486 laboratory-confirmed cases. There have been 3,442 statewide, and 100 people have died from the virus.

But the gulf between laboratory-confirmed tests and the sewage estimate is not entirely unexpected.

It's likely the number of people that have tested positive is far fewer than those who have actually had – and could have spread – the virus.

Laboratory tests are limited. So nobody knows how many people may have been a carrier but had mild symptoms and were never were tested. Looking at the presence of the virus in local sewers could provide estimates to that.

"This is a new technology," said New Castle County Executive Matt Meyer, whose administration partnered with the testing company. "Though it is an estimate, it is helpful to us."

Meyer said the intent is to conduct further tests to compare with the April 14 sample.

Such information could help reveal hot spots around the county where officials can direct more testing of people who are not showing symptoms, Meyer said.

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That information could also be a tool for officials trying to gauge how many new cases and hospitalizations Delaware will continue to face and – somewhere down the line –when to ease restrictions on public life.

While the high estimated prevalence of the virus may strike alarm at first glance, Meyer said it could also be a measure of how close the county is to reaching "herd immunity," a point where most residents are immune to the virus, slowing its spread.

That's usually reached through a vaccine, which does not currently exist. And the science is unclear now whether those exposed become entirely immune and how long they will remain immune. Some reports have indicated some people may become reinfected.

Meyer said it could be another tool in making decisions in an uncertain future, but will not by itself lead to any "grand decisions."

The study was conducted by a Massachusetts Institute of Technology startup Biobot Analytics. Since March 23, the startup has gathered samples from more than 100 treatment facilities in about two dozen states.

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The county's results are similar to those in Los Angeles County, California, where 4 percent of the population were estimated to have been exposed.

The results also track with a series of preliminary tests in the county that determines whether or not someone had the virus by measuring antibodies in their blood, said New Castle County Public Works Stormwater and Environmental Program Manager Mike Harris.