The Centers for Disease Control and Prevention launched a new dashboard Friday that tracks COVID-19 in wastewater samples across the country.

The data comes from the federal agency’s National Wastewater Surveillance System, which connects more than 400 sites across 28 states and the District of Columbia.

More than 34,000 samples have been collected representing 53 million Americans, Dr. Ann Kirby, program lead for the CDC’s National Wastewater Surveillance System, said during a media briefing.

Over the last 15 days, the dashboard shows that 98% of treatment facilities have detected the virus in all of their samples.

However, 70% of all facilities say the amount of virus found in samples has decreased compared to two weeks ago — a sign that COVID-19 cases are on the decline.

Between 40% and 80% of COVID-19 patients shed genetic material from the virus, or viral RNA, in their feces.

When stool is flushed down the toilet, it flows through a drainage system into a treatment facility, where it becomes part of wastewater.

The same tests that are used to determine if someone is positive — what are known as PCR tests — can also detect the virus in wastewater samples.

During the media briefing, Kirby said wastewater surveillance provides public health officials with “a better understanding of COVID-19 trends in communities.”

Because people shed the virus when they are in the early stages of infection, increases in levels of viral RNA in wastewater are often seen before the number of cases rise.

This makes wastewater an early warning system of sorts and helps predict where COVID-19 outbreaks are going to occur.

“These can inform important public health decisions such as where to allocate mobile testing and vaccination sites,” Kirby said. “Public health agencies have also used wastewater data to forecast changes in hospital utilization, providing additional time to mobilize resources in preparation for increasing cases.”

Dr. John Brownstein, an epidemiologist at Boston Children’s Hospital and an ABC News contributor, said wastewater samples helped predict the omicron wave before it even hit.

“Well ahead of when we knew this omicron wave was creating this massive increase in cases, we saw the signal in the wastewater, and the sort of scale of amount of virus that was detected in wastewater was far greater than any other point in the pandemic,” said Brownstein, who is a member on the board of advisors of wastewater analytics company Biobot.

This is not the first time that wastewater surveillance has been used to track public health concerns.

Several countries overseas have used the tool to monitor polio outbreaks. In many European cities, public health officials have used wastewater surveillance to track opioid use.

Brownstein said wastewater has even been used to track the flu and can continue to be useful when COVID becomes a more seasonal, endemic disease.

“It can be absolutely used to look at early signals of any viral disease,” he said. “I think wastewater can be part of the public health fabric for just general surveillance. While COVID may become more sort of an endemic, seasonal virus, having a window into when we may see surges — especially with new variants — will be super critical.”

However, wastewater surveillance has some limitations. About one-fifth of U.S. households are not connected to a public sewer and use septic systems instead, according to the Environmental Protection Agency.

This means wastewater testing would not be able to detect viral spread in 20% of American homes.

Additionally, Kirby said the testing cannot determine if a community is free from infection, so it’s best if it’s used in conjunction with other tools such as case-based surveillance.

But, if Americans do start to see rates of viral RNA in wastewater increase, they can implement the same measures they would use if cases were rising, just earlier.

“You want to take all the same actions: masking, distancing, getting vaccinated if you’re not, testing if you’re feeling sick,” Kirby said. “But with wastewater, you can start doing those a few days earlier and those extra days can really make a difference in the ultimate trajectory of that surge in your community.”