Officials are exploring other possibilities, too. As part of the C.D.C.’s Traveler Genomic Surveillance program, for instance, Ginkgo Bioworks, a Boston-based biotechnology company, is now testing wastewater samples from planes landing at the international terminal at San Francisco International Airport.

“Putting in place these indirect mechanisms that can give you a sense of what’s going on in the world are really important, as other forms of testing start falling off,” said Andrew Franklin, the director of business development at Concentric by Ginkgo, the company’s biosecurity and public health arm.

The American Rescue Plan has provided enough funding to conduct wastewater surveillance in all states and territories through 2025, Dr. Kirby said.

But maintaining wastewater surveillance will require ongoing funding over the longer term, as well as continued buy in from local officials, some of whom might lose interest as the emergency phase of the pandemic winds down. “We’re going to see some fatigue-based dropouts,” said Guy Palmer, an infectious disease pathologist at Washington State University and the chair of the wastewater surveillance committee of the National Academies of Sciences, Engineering and Medicine.

So proponents of wastewater surveillance are hoping to demonstrate its continued utility, both for Covid-19 and other diseases. Some jurisdictions are already using wastewater to track influenza and other pathogens, and the C.D.C. hopes to roll out expanded testing protocols by the end of the year, Dr. Kirby said.

“This is part of our surveillance portfolio for the long haul,” Dr. Kirby said. “I think we’re really going to see how powerful it can be once we’re out of this emergency response period.”