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Elsewhere in the world, countries have been expanding their use of sewage sampling. Here are some of their key findings.

Every morning at 5, a group of scientists in Ottawa receives samples of the previous day’s sewage to test for traces of the coronavirus in wastewater pooled from “over a million souls,” said Alex MacKenzie, a senior scientist at the CHEO Research Institute.

MacKenzie is part of a team at CHEO and the University of Ottawa that piloted the program, which Ottawa’s public health department uses to provide daily reports on the coronavirus’s spread. In early October, MacKenzie’s team reported that concentrations of the virus in the area’s wastewater had doubled in the past month and increased tenfold since June.

Other cities and provinces across Canada, as well as several universities, are watching sewage systems for signs of the virus.

Such testing provides crucial information at little cost, MacKenzie said. Each sample costs only several hundred dollars and gives a reliable snapshot of the big picture, and there are few privacy concerns because the tests do not detect individual infections.

“There’s a general realization that we should be doing this as much as possible,” he said.

The model could have applications for vulnerable populations, he said, such as those living in homeless shelters, prisons and elder-care homes, if plumbing systems in buildings or apartment blocks were frequently tested.

While many countries and cities test wastewater for virus RNA, MacKenzie said his team is also looking for the protein that surrounds the genetic matter. The virus’s RNA, he said, “is a fragile beast,” while the protein is sturdier and could provide an even more accurate picture of the virus’s spread.

In early February, over a month before the global pandemic was declared, scientists from the Dutch KWR Water Research Institute in Nieuwegein began periodically testing sewage samples from seven cities and one airport, according to a paper they published in July. Initial tests came back negative. But on March 5, Dutch scientists detected the coronavirus in wastewater at a treatment plant in the city of Amersfoort, about 32 miles southeast of Amsterdam. Weeks later, Amersfoort’s first case of the virus was confirmed.

Since March, the Dutch National Institute for Public Health and the Environment has been taking weekly samples from sewage systems across the country. As in the first study, researchers were able to detect small traces of the virus before cases or outbreaks were confirmed. While the program began with a few dozen sites, since Sept. 7 the institute has been testing systematically each of the country’s more than 300 sewage treatment centers, according to its website.

During the 2003 SARS outbreak, researchers in Hong Kong were puzzled by the question of how the virus had infected hundreds of people in a 33-story apartment building. Their conclusion: SARS, a coronavirus related to the one causing covid-19, had probably aerosolized and spread through the plumbing system. Scientists narrowed in on a patient zero, who had diarrhea in