Public-health experts traditionally track the spread of an infectious disease through clinical data such as test results, hospitalizations and deaths. As Covid-19 continues to spread, scientists are turning to an alternative measure: wastewater analysis. SARS-CoV-2, the virus that causes Covid-19, can be shed in an infected person’s feces. By sampling sewage at waste-treatment plants, scientists can get a picture of how widespread Covid-19 has become in a community, and how its prevalence changes over time.

The extent of Covid-19 testing has varied throughout the pandemic. Because wastewater can be sampled at regular intervals, it may provide a reliable adjunct to data from clinical tests.