Researchers don’t yet have a definitive answer. Converting wastewater data into an estimate of positive cases requires a key metric scientists are still learning about: how much virus an infected person sheds. That can depend on whether the person is sick—and, if so, the stage of illness the person is in and how severe the illness is. Because those variables are hard to nail down, Wiedenheft has intentionally avoided reporting such estimates. “We don’t feel confident enough to make that translation,” he says.
Read: What happens when there’s sewage in the water?
But in some cities, officials have released estimates using wastewater data to indicate how many community members are infected. For instance, analyses by Biobot, a wastewater-testing startup, estimated that levels of SARS-CoV-2 found at one point in Moscow, Idaho’s sewage corresponded to 1,800 cases. At the time of that estimate, Latah County, where Moscow is located, had only 46 known cases. (Biobot declined to be interviewed, and its hired communications firm didn’t answer questions about how the company calculates case estimates. A recent study published by Biobot’s co-founders and colleagues reported that assumptions about individuals’ viral load can massively affect these estimates. According to their calculations, the assumption that infected people have a low viral load leads to an estimate that 5 percent of the population is infected; assuming a high viral load will lower that estimate to 0.1 percent.)
So far, many local officials have looked to wastewater analyses more as a way to corroborate their knowledge about community viral spread than as a way to estimate case numbers. “We won’t know how many people in the community have COVID-19 from the methodology, but this data will tell us if trends are going up or down,” says Nicole Rowan, the clean-water program manager at the Colorado Department of Public Health and Environment, which has launched a statewide wastewater-testing effort.
Matt Kelley, a health officer at the Gallatin City-County Health Department in Bozeman, says that when Gallatin County went through a period of few cases in May, the sewage data gave additional confirmation that spread had slowed. “It was somewhat reassuring to have another backstop indicator of what was happening,” he says—and when positive COVID-19 swab tests rose again, so did levels of SARS-CoV-2 in the water. Kelley notes that once cases fall again, it will be helpful to track wastewater as an indicator of undetected community spread. “If we’re not seeing tests in the traditional medical testing, and also not seeing them in the wastewater, that’s a validating factor for us.”
As more areas launch wastewater-tracking programs, Wiedenheft says one thing is still needed: a central repository for this data, which could provide a bigger-picture view of viral spread. Over the last few months, Montana has developed several monitoring sites, and some states, like California, Colorado, and Wyoming, have created their own networks. It’s difficult to compile a list of cities conducting testing, since some key data are private; Biobot says it’s working with 400 facilities, but declined to provide additional information, including how many cities that corresponds to. Wiedenheft points to Johns Hopkins’ popular coronavirus tracker as an example of a well-organized health surveillance tool. “It would be nice to have a wastewater-surveillance website that does the same thing,” he says, “where you could look at a geographic map and look at what’s happening.”