Sewage may offer early signs of coronavirus outbreaks.

People infected with the coronavirus may shed extremely high amounts of virus in their stool even before they show symptoms — if they ever do — suggesting that testing wastewater may offer health officials a way to spot budding community outbreaks early, researchers have found.

Scientists at M.I.T. and elsewhere compared coronavirus concentrations in sewage from an urban treatment facility in Massachusetts with Covid-19 cases in the same area and found that changes in coronavirus levels in wastewater preceded rises and falls in positive test results by four to 10 days.

Their study has yet to be peer-reviewed, but the findings, along with those in a study published in the October issue of Nature Biotechnology by Yale researchers, suggest that sewage surveillance could play an important role in helping contain the pandemic.

The practice might give public health officials warning about infection upticks perhaps a week earlier than clinical-testing data alone can. That means that they could issue health advisories or order closings sooner, giving those measures a better chance of working.

Had such monitoring been available early in the pandemic, heath officials might have realized sooner that the virus was spreading in communities on both coasts and been better able to forecast where emergency medical workers and scarce supplies like personal protective gear and ventilators would be needed.

“You would want any additional information you could get about the severity and location of the virus,” said Ted Smith, an associate professor of medicine and director of the Center for Healthy Air, Water and Soil for the Christina Lee Brown Envirome Institute at the University of Louisville

Early diagnostic testing was so limited, Dr. Smith said, that “our ability to see the prevalence of the virus was really compromised.”