Communities around the world have embraced the trend of wastewater monitoring to determine whether COVID-19 is circulating. From the start, the approach promised an inexpensive way to identify an outbreak early, giving public health authorities time to take action. Viral particles are shed in feces even before infected people show symptoms, and samples drawn from the sewer are a snapshot of the health of everyone served by that system. Researchers hoped regularly testing those samples would provide officials data to better allocate resources.

Now, some 18 months into the pandemic, researchers have a better idea of what sewage surveillance can and can’t tell us about a local outbreak—and how the emergence of variants complicates the measurement and interpretation of wastewater data.

The clearest takeaway is that widespread efforts to measure the virus in wastewater have proven the method to be a reliable way of monitoring for COVID-19 outbreaks. “There’s plenty of evidence that we have early-warning capacity to see what’s going to happen with respect to recorded infections, morbidity, and mortality,” says Rolf Halden, an environmental engineer at Arizona State University and cofounder of AquaVitas, a wastewater-based epidemiology start-up that performed the first nationwide study in over 100 US cities for the Department of Health and Human Services.

In the early days of the pandemic, people hoped that wastewater measurements could be a proxy for widespread clinical testing and reveal the number of COVID-19 infections in a community. The reality is that extracting case numbers from wastewater data continues to be a challenge, says Amy E. Kirby, program lead for the National Wastewater Surveillance System (NWSS) at the US Centers for Disease Control and Prevention. Researchers can calculate case numbers from sewage samples, but their wide confidence intervals make it hard to say anything definitive. “Yeah, you get a number, but the actual true range is so big that it’s encompassing any kind of changes over time,” Kirby says.