“The flu is pretty tricky to test in wastewater,” said Prof. Alm, co-director of MIT’s Center for Microbiome Informatics and Therapeutics. “For Covid-19, we’re fortunate that it is excreted in large enough amounts.”

Scientists around the world have long used the technique, known as wastewater-based epidemiology, to track social scourges such as drug use or chemical spills with the aim of guiding policy decisions. An outbreak of polio in Israel in 2013 was detected by the monitoring of sewage.

Now, scientists are hopeful that sewage can bolster surveillance efforts for the coronavirus as countries look to end lockdowns that have hobbled their economies. Unlike clinical tests such as nasal swabs that return results only of individuals, they believe wastewater samples can provide information on hundreds of thousands of people or more in one hit.