More than a year after the Centers for Disease Control and Prevention established its National Wastewater Surveillance System, the agency added wastewater data to its Covid-19 data tracker on Friday, providing a quick glimpse of whether the levels of coronavirus are rising or falling in hundreds of communities across the country.

People who are infected with the coronavirus shed the virus in their feces, and analyzing wastewater can help officials estimate how prevalent the virus is in a particular community and which variants are circulating.

And because people with the virus often begin shedding it before they seek care or testing — if they ever do — it can also provide an early warning system of coming surges or the circulation of new variants. Several communities detected Omicron in wastewater before any test samples from infected residents showed they had the variant.