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COVID wastewater signals decreasing across Toronto

The Star’s signal map uses data sourced from each GTA public health unit and pulled into one location, showing trends in sewersheds or wastewater catchment areas.
May 6, 2022
May Warren and Kenyon Wallace

With Ontario’s mask mandate in the rearview mirror and a lack of widespread provincial testing, you may be wondering about the COVID-19 spread in your community.

In the absence of any official public database of COVID signals in the GTA’s sewage, the Toronto Star brings you its own COVID wastewater signal map to give you a sense as to whether cases are increasing, decreasing or remaining stable in your area.

The Star’s signal map uses data sourced from each GTA public health unit and the Ontario COVID-19 Science Advisory Table showing trends in sewersheds or wastewater catchment areas for each treatment plant across the region.

This week, of 20 GTA plants analyzed, COVID wastewater signals are increasing at only one (Port Darlington in Durham) while levels are stable at 12 and decreasing at seven.

According to the latest Toronto data, wastewater signals at all four wastewater treatment plants in the city are decreasing.

For the city of Toronto, there’s a slightly different framework that still follows the stoplight approach. The Ministry of the Environment, Conservation and Parks and the science table agreed on a slightly different set of criteria for the city after their risk assessments differed: green signifies little change or a decrease, yellow means caution as slight increases are observed and red means a larger increase in signal is occurring.

Provincewide wastewater signal analysis from the Science Table show that cases are beginning to drop, after hitting a plateau around the 100,000 mark and then showing a bump after the Easter weekend.

Wastewater surveillance has become the most accurate method for determining daily cases since the province suspended widespread polymerase chain reaction (PCR) testing at the end of last year.

“Prior to the Omicron wave, we had reasonably good testing in the province ... that still gave us a good indication of what the peaks and troughs were,” said Eric Arts, Canada Research Chair in Viral Control and an immunology professor at Western University. “Now we don’t have that at all.”

Wastewater surveillance monitors the genetic fingerprint of SARS-CoV-2, the virus that causes COVID, in water carrying fecal matter that we flush down our toilets. The amount of fragments of the virus’s RNA detected in wastewater tells scientists whether cases are going up, down or staying stable in a particular wastewater catchment area.

“It has a good home, both in the intestines of people -- the colon -- and when it’s released from us into the wastewater,” Arts noted.

Robert Delatolla, a professor at the University of Ottawa whose lab tracks the virus in Ottawa wastewater, said the indicator has never been more important, particularly as COVID trends will show up there before they hit hospitals.

“It’s sort of like a smoke alarm for health resources.”