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...
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