Testing sewage offers useful data on the spread of Covid-19.

Although Covid-19 is primarily a respiratory disease, research conducted early in the pandemic revealed that people infected with the coronavirus often shed it in their stool. This finding, combined with the scale and urgency of the crisis, spurred immediate interest in tracking the virus by sampling wastewater.

In the past year, many scientists have been drawn into the once niche field of wastewater epidemiology. Researchers in 54 countries are tracking the coronavirus in sewage, according to the Covid19Poops Dashboard, a global directory of the projects.

These teams have found that the wastewater data seemed to accurately indicate what was happening in society. When the number of diagnosed Covid-19 cases in an area increased, more coronavirus appeared in the wastewater. Levels of the virus fell when areas instituted lockdowns and surged when they reopened.

Several teams have also confirmed that sewage can serve as an early warning system: Wastewater viral levels often peaked days before doctors saw a peak in official Covid-19 cases.

And wastewater analysis has allowed scientists to detect the arrival of certain variants in a region weeks before they are found in people — and to identify mutations that have not yet been detected in people anywhere.

The surveillance is not a replacement for clinical testing, experts said, but can be an efficient and cost-effective complement. The approach is likely to be especially valuable in low- and middle-income countries, where testing resources are more limited.

“Not every population gets tested, not everyone has access to health care,” said Dr. Marc Johnson, a virologist at the University of Missouri. “If there’s groups of people that are asymptomatic, they probably aren’t getting tested either. So you aren’t really getting the full big picture. Whereas for our testing, everyone poops.”

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