Rising levels of COVID-19 detected in Toronto wastewater as province drops mask mandate

As Ontario moves to drop mandatory masking in public settings, wastewater surveillance levels across the province continue to show high, albeit stable, numbers of COVID-19 infections, but experts caution it’s too soon to see the impact of the recent lifting of public health measures .

In the absence of widespread polymerase chain reaction (PCR) testing, wastewater surveillance is now Ontario’s best early indicator in the tracking of COVID spread.

At any other time of the pandemic, what wastewater surveillance currently shows would be alarming: an estimated 15,000 to 20,000 new infections a day, according to provincewide analysis by the Ontario COVID-19 Science Advisory Table. This is four times higher than where daily cases were at the height of the second wave last April, and around where they hit in late December, while the province was still doing widespread PCR testing.

“We do not yet know what impact the last reopening step had on case numbers or changing people’s behaviour,” said Dr. Peter Jüni, epidemiologist and scientific director of the science table. “If we want evidence-based decision making, we need to wait at least 10 more days before we could be confident that we know what’s happening with COVID infections by looking at wastewater levels.”

In Torontothe most recent data from Feb. 6 to March 7 shows COVID wastewater levels are rising in the Ashbridge’s Bay and Highland Creek plant catchment areas, but stable for Humber and North Toronto. The overall province-wide wastewater signal is down significantly since the beginning of January, but appears to have plateaued.

On Wednesday, the province announced it is scrapping mandatory masks for all indoor settings as of March 21, except for on public transit, in hospitals, long-term-care homes and other congregate settings. Mask mandates for these latter places will be dropped on April 27.

In his last scheduled weekly pandemic address Wednesday, Dr. Kieran Moore, the chief medical officer of health, said vulnerable people should continue wearing masks.

“Removing the mask mandates does not mean the risk is gone,” he told reporters at Queen’s Park. “It is a choice now.”

Asked about surveillance of the virus now that PCR tests pick up only a fraction of actual cases in Ontario, Moore said he is confident in the province’s strategy, pointing to hospitalizations, deaths and the “septic” surveillance available on the science table’s website.

All the public health units in the GTA, except hamiltonalso publish wastewater surveillance data on their websites, broken down by treatment plant catchment areas.

On New Year’s Eve, as Omicron overwhelmed the testing system, the province restricted access to PCR tests to only high-risk settings and people.

Without these case numbers to rely on for an accurate picture of community spread, testing for COVID’s genetic fingerprint in wastewater, which began in Canada in April 2020, has moved to the forefront.

Fortunately the PCR eligibility changes don’t “affect how many people go to the bathroom,” said Robert Delatolla, a professor at the University of Ottawa whose lab tracks COVID in Ottawa wastewater.

Wastewater surveillance works by monitoring the level of SARS-CoV-2, the virus that causes COVID, in water carrying fecal matter after we flush our toilets. Samples are taken at sites throughout the province, providing an early indicator of whether cases are going up, as virus fragments will be detected in feces before someone is sick enough to go to the hospital.

“It really can tell us, are we out of the woods? How are we doing? Delatolla said. “It gives you that piece of information where you can make those choices for yourself, and it fills in that void that we have now.”

Claire J. Oswald, an associate professor in the department of geography and environmental studies at Ryerson University who’s working on samples from Toronto’s Humber Wastewater Treatment Plant, said there may be some “ups and downs” now that the overall levels of COVID have dropped from the Omicron peak.

Oswald credits the labs that do the wastewater testing and the province for organizing them, calling it a “success story” that “hopefully can be leveraged for other things beyond COVID,” such as tracking of flu cases.

“For people who wonder, ‘does this really work?,’ it does,” said Dr. Eric Arts, Canada Research Chair in Viral Control and an immunology professor at Western University. “The virus stays relatively stable in the wastewater, so we can monitor it. It tracked almost perfectly with cases when we were monitoring cases.”

Arts, along with other researchers at other universities, is monitoring wastewater for the province and the Public Health Agency of Canada. He noted that this type of surveillance likely provides a better representation of total cases in Ontario simply due to the fact that a lot of infected people aren’t getting tested.

“Now we can pinpoint which regions might have an uptick in transmission,” he said. “If we needed to, we could be very specific in putting stronger public health measures in a specific region.”

Jüni says he is concerned that if Ontarians start to embrace the newly allowed freedoms, at the same time that we are dropping masking mandates, cases could start going up again.

“This could be too much in terms of high-risk contacts and transmission,” he said. “We first want to make sure wastewater levels stay relatively stable, or if it does go up, stabilizes again. The longer we wait, the closer we get to spring, and the more the good weather will help.”


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