In late 2020, COVID-19’s global death toll was rising as cold weather in the Northern Hemisphere and holiday gatherings spurred rapid transmission of SARS-CoV-2 in the absence of a vaccine. Scientists and public-health officials were desperate for new ways to track the virus, which often moved faster than contact tracers could follow it.
Tong Zhang, an environmental engineer and microbiologist at the University of Hong Kong (HKU), and his colleagues were pioneers of what was fast becoming a popular surveillance method. They had been collecting periodic wastewater samples from about two dozen maintenance holes in the city and testing the sewage for coronavirus DNA, with support from Hong Kong’s government. In late December, they traced an outbreak to a single apartment building where there had been no sign of cases1.
The government quickly took action. Officials tested all of the building’s 2,000-odd residents; 9 tested positive. “Those people were isolated and went to a quarantine site. So they stopped the transmission chain,” Zhang says. After that success, he and his colleagues expanded their efforts.
Wastewater testing remains part of Hong Kong’s COVID-19 strategy to this day. A Hong Kong government team tests for the coronavirus at about 20 sites across the city each week, Zhang says, and he and his colleagues have expanded the analysis of these samples to cover other pathogens, including influenza, rotavirus, norovirus and mpox, as well as markers of antimicrobial resistance. He views wastewater testing as a way to gauge the health of an entire community at once. “If we can make the methodology more standardized”, this tool becomes a “promising and exciting” way to screen the world for pathogens, including those that scientists haven’t yet identified,he says.
Many researchers are following similar approaches. There are currently more than 4,600 sites around the world where wastewater is being collected for SARS-CoV-2 testing, and some of the research teams involved are investigating other potential applications, such as tracking illicit drug use and even the prevalence of cancer.
But whether this has the potential to be an effective public-health strategy is still a matter of debate. Leo Poon, a colleague of Zhang’s at HKU’s School of Public Health, says that more research should be done before health agencies expand their sewage testing programmes and make this surveillance part of their routine budgets. “There’s still a lot unknown,” he says, particularly in terms of testing for pathogens besides SARS-CoV-2. “I think there’s a steep learning curve at the moment: when we detect something, what does it mean?”