According to the CDC, 80 percent of households in the United States have their wastewater go to a municipal treatment plant, which it can reach in a matter of hours. Testing wastewater can therefore track the virus in almost real time. Infected people often start shedding virus days before they have any symptoms, if they develop symptoms at all. Collected wastewater can carry COVID before the person who shed it even knows they are sick. That means that wastewater testing can serve as a very effective early warning system, said Freedman, helping to alert public health officials of future surges or ascendant variants.

That early shedding is “the biological characteristic of COVID [that] actually makes wastewater testing effectively a good way of solving how the virus evolves and how the virus spreads in the community,” he said.

Another advantage to wastewater testing is that it’s passive, said Katelyn Jetelina, an epidemiologist at the University of Texas Health Science Center School of Public Health who writes the newsletter Your Local Epidemiologist. “You’re captured irrespective of your testing behavior,” she said. With wastewater testing, said Jetelina, as long as you flush your toilet, your data is picked up.