Each sample collected after the initial US case was reported tested positive at “significantly higher” levels than expected based on clinically confirmed cases in the state. Wanna Thongpao/Shutterstock

An unexpectedly high presence of the novel coronavirus has been detected in a Massachusetts sewage treatment facility, providing a potential new way to determine community case numbers without widespread testing.

“Despite [the] pandemic spread of SARS-CoV-2 worldwide, broad access to testing in the United States has thus far been severely limited,” write the study authors in a paper not yet peer-reviewed on the pre-print server medRxiv. "While it is impractical to test every US resident for SARS-CoV-2, the virus has been found in the stool of confirmed COVID-19 patients, making it a promising candidate for wastewater-based epidemiology (WBE).”

WBE is a methodology used to detect the presence of pathogens in communities and to estimate how common the virus may be without requiring each individual to be tested. Such measures were adopted during China’s 2003 SARS-Cov outbreak, a closely related virus to SARS-CoV-2 that caused severe acute respiratory syndrome, and to track diseases in Israel, Egypt, and Sweden.

To determine whether WBE would be a good candidate to track SARS-CoV-2 in the hard-hit state of Massachusetts, researchers from Biobiot Analytics and the Massachusetts Institute of Technology collected 10 samples of sewage wastewater from a major urban treatment facility for comparison against water samples collected from before the first US case was documented on January 20. All of the samples before tested negative for the virus, whereas each sample collected between March 18 and 25 tested positive at “significantly higher” levels than expected based on clinically confirmed cases in the state.

Estimates suggest that about 5 percent of all fecal samples tested were positive, while only 0.026 percent of the population were confirmed with the virus. It is not yet clear why the discrepancy is so high, but it could depend on how much of the virus people are shedding or perhaps account for asymptomatic carriers.

The researchers suggest that the approach is scalable and could be used to model the SARS-CoV-2 pandemic, as well as future outbreaks in cities around the world, particularly in places where in-person testing may not be available.

“Wastewater surveillance may represent a complementary approach to measure the presence and even prevalence of infectious diseases when the capacity for clinical testing is limited,” write the researchers. “Moreover, aggregate, population-wide data can help inform modeling efforts.”

Further understanding of the presence of the virus at the population level could also help government and hospital officials to implement informed policy measures.