“There is real hope that this can be a sensitive, early warning” if, as officials ease social distancing measures, Covid-19 begins to spread again, said Peter Grevatt, CEO of the nonprofit Water Research Foundation. “Several labs have achieved a proof-of-concept in terms of demonstrating the ability to detect the RNA [genetic material] of the virus in wastewater.” Studies in the U.S. and the Netherlands, among others, have shown you can pick up a signal about a week before the first clinical case…

The stakes are enormous. As states ease social distancing, they need to know if they are reopening too quickly. If they can’t detect a surge of cases until patients show up in emergency rooms, it could be too late to prevent a repeat of this spring, when hospitals in New York City and elsewhere were overwhelmed. If, as many hope, the country makes it through the summer with a “flattened curve” — keeping cases below the number that hospitals can handle — then it will be crucial to be on high alert for any second wave in the fall.