As winter’s chill sets in across Canada, the race to the indoors begins, often corresponding to a surge in respiratory infections. With viruses thriving in close quarters, people are at an elevated risk of being infected with these illnesses during the colder months. You can protect yourself from respiratory viruses by following Government of Canada recommendations.
The flu flows in winter, ebbs in spring
Wastewater monitoring can provide a snapshot of the activity level of respiratory diseases in a community because infected people shed these viruses into the wastewater. Even if a person doesn’t have symptoms, they may still shed viruses which can be detected by testing the wastewater. Analyzing data from the Canadian Wastewater Survey of five Canadian cities from the summer of 2024 until early fall of 2025 highlights the ebb and flow of the activity levels of various respiratory diseases such as COVID-19, the flu and respiratory syncytial virus (RSV) throughout the flu seasons (see “Why we monitor wastewater” section).
The concentrations of flu and RSV viruses were higher during the winter of 2025 in Halifax, Montréal, Toronto, Edmonton and Metro Vancouver, five of the cities covered by the Canadian Wastewater Survey. Like the snow, flu and RSV activity increased to high levels early in the winter months and began decreasing around March (see “High, low and in between” section).
From the summer of 2024 to the fall of 2025, flu and RSV virus activity levels in all five cities matched this general seasonal trend. In all five cities, flu virus activity levels peaked in January or February 2025.
Flu and respiratory syncytial virus levels in wastewater in the fall of 2025
In September 2025, flu and RSV concentrations in wastewater were moderate to low across all five cities, in line with levels during the same month a year earlier.
COVID-19 levels in wastewater
Although COVID-19 is also a respiratory disease, it shows more variability and less consistency in seasonal patterns of virus activity compared with the flu or RSV.
Indeed, from the summer of 2024 to the fall of 2025, COVID-19 wastewater virus activity peaked at different times across the five cities studied.
For example, COVID-19 virus activity levels peaked in Metro Vancouver in the summer (mid-August 2024).
In Edmonton and Halifax, COVID-19 virus activity levels were highest in the fall (October and December 2024).
In Montréal and Toronto, COVID-19 virus activity levels were highest during the winter (December 2024 to February 2025).
There was an uptick in COVID-19 virus activity levels from August 2024 to mid-September 2024 across all five cities surveyed, suggesting the potential for increased transmission during the back-to-school season.
COVID-19 levels in wastewater in the fall of 2025
COVID-19 continues to be present in Canada. Wastewater testing in September 2025 revealed that COVID-19 virus activity levels in Metro Vancouver were fluctuating between high and moderate, similar to September 2024.
In Edmonton, there were high virus activity levels of COVID-19 at the beginning of September 2025 and then moderate levels for the rest of the month, similar to trends observed in September 2024.
COVID-19 virus activity levels were high in Toronto during the first week of September 2025. Conversely, a year earlier, COVID-19 virus activity levels were moderate throughout all of September.
In Montréal, COVID-19 virus activity levels were moderate throughout September 2025, while higher levels had been observed in the same month a year earlier.
COVID-19 virus activity levels in Halifax were moderate at the beginning of September 2025 and higher toward the end of the month. A similar pattern was observed in September 2024.
Keep tabs on wastewater virus activity levels all winter and throughout the year
While this story focused on virus activity levels in five cities across Canada, wastewater data are available for the Canadian Wastewater Survey on the Public Health Agency of Canada’s InfoBase dashboard and includes wastewater data for regions across Canada.
For more information on COVID-19, flu and RSV levels in your area, check out the Wastewater monitoring dashboard, which is updated twice per week.
Why we monitor wastewater
Wastewater monitoring can provide estimates of the level infectious diseases, drugs and antimicrobial resistance in a community.
It has the potential for generating near real-time data on geographical and temporal trends without the need for self-reported surveys. It can also be used as an early warning system and a tool to monitor the effectiveness of public health measures.
Statistics Canada, in partnership with the Public Health Agency of Canada’s National Microbiology Laboratory, continues to collect data for the Canadian Wastewater Survey to better understand the levels of SARS-CoV-2 (the virus that causes COVID-19), flu and RSV in the wastewater of the following municipalities: Metro Vancouver, Calgary, Edmonton, London, Peel Region, Toronto, Kingston, Montréal, Moncton and Halifax. This article is based on observations of the data for five cities with adequate historical data that are available for analysis.
High, low and in between
Thresholds to define low, moderate and high virus activity levels are determined by calculating running percentiles based on a year’s worth of data back from the datapoint in question for a given municipality. High values are those above the 75th percentile, low values are below the 25th percentile and moderate values are between those percentiles.
Contact information
For more information, contact the Statistical Information Service (toll-free 1-800-263-1136; 514-283-8300; infostats@statcan.gc.ca) or Media Relations (statcan.mediahotline-ligneinfomedias.statcan@statcan.gc.ca).