Although Covid-19 is primarily a respiratory disease, research conducted early in the pandemic revealed that people infected with the coronavirus often shed it in their stool. This finding, combined with the scale and urgency of the crisis, spurred immediate interest in tracking the virus by sampling wastewater.
By searching for, and then counting, certain coronavirus genes in sewage, researchers hoped to determine whether the virus was present in a particular region and how widespread it was. Before long, wastewater surveillance projects were popping up everywhere from Kansas City, Mo., to Kathmandu, Nepal.
The resulting data, now appearing in a flood of scientific papers and preprints, have provided a powerful proof of principle. Scientists have detected the virus in all kinds of environments: in treated and untreated water, in sludge and settled solids, in sewers and septic tanks, in pit latrines and open drainage systems. They found it in water flowing into enormous treatment plants and out of schools, dormitories and nursing homes. “It’s just fascinating how robust this tool has become,” said Peter Grevatt, chief executive of the Water Research Foundation.
Teams all over the globe — in the United States, France, Portugal, India, Iran, Brazil, Canada and elsewhere — also found that the wastewater data seemed to be an accurate indicator of what was happening in the real world. When the number of diagnosed Covid-19 cases in an area increased, more coronavirus appeared in the wastewater. Levels of the virus fell when areas instituted lockdowns and surged when they reopened.
Multiple teams have also confirmed that sewage can serve as an early warning system; Wastewater viral levels often peaked days before doctors saw a peak in official Covid-19 cases.
This lead time, which can range from a couple of days to two weeks, depends partly on the robustness of local clinical testing programs, scientists say: When more people are being tested for the virus more frequently, the wastewater data provides less advance warning. The lead time also exists because infected people often begin shedding the virus, SARS-CoV-2, before they feel symptoms and then, once they fall ill, frequently delay seeking medical care.
“I think wastewater has proven itself as one of the most, I would say, objective means of understanding what SARS-CoV-2 is doing in our society,” said Gertjan Medema, a microbiologist at KWR Water Research Institute in the Netherlands.