Although Covid-19 is primarily a respiratory disease, research early in the pandemic found that people infected with the coronavirus frequently toss it in their stools. This finding, coupled with the magnitude and urgency of the crisis, immediately sparked interest in tracking the virus by sampling wastewater.
By finding and then counting specific coronavirus genes in wastewater, the researchers hoped to determine if the virus was present in a particular region and how widespread it was. It wasn’t long before sewage monitoring projects started popping up all over Kansas City, Missouri, until Kathmandu, Nepal.
The resulting data, which now appears in a flurry of academic papers and preprints, has provided strong evidence of the principle. Scientists have detected the virus in all sorts 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 that ran into huge wastewater treatment plants and out of schools, dormitories, and nursing homes. “It’s just amazing how robust this tool has become,” said Peter Grevatt, executive director of the Water Research Foundation.
Teams around the world – in the US, France, Portugal, India, Iran, Brazil, Canada and elsewhere – also found that the sewage data appeared to be an accurate indicator of what was happening in the real world. As the number of diagnosed Covid-19 cases increased in an area, more coronavirus appeared in wastewater. Virus levels fell when areas were closed and increased when they were reopened.
Several teams have also confirmed that wastewater can serve as an early warning system. Virus concentrations in wastewater often peaked days before doctors saw a peak in official Covid-19 cases.
This lead time, which can range from a few days to two weeks, depends in part on the robustness of local clinical testing programs. Scientists say, if more people are tested for the virus more often, the sewage data will offer less warning. The lead time is also because infected people often start shedding the SARS-CoV-2 virus before they experience symptoms and then often delay seeking medical care once they get sick.
“I think wastewater has proven to be one of the most objective means of understanding what SARS-CoV-2 is doing in our society,” said Gertjan Medema, a microbiologist at the KWR Water Research Institute in the Netherlands.