SARS-CoV-2 levels detected in wastewater across five Canadian cities: Current trends, January 2023 to February 2025
Released: 2025-04-03
Monitoring respiratory illness trends is important for robust public health planning and reducing societal and healthcare burdens. While influenza and respiratory syncytial virus typically follow a seasonal pattern, peaking in activity from November to April, SARS-CoV-2 is less predictable. Recent wastewater data reveal that viral SARS-CoV-2 concentrations sometimes rise outside of flu season. Additionally, SARS-CoV-2's high transmissibility and rapid evolution, which affects immunity, keep the risk of infection present year-round.
Statistics Canada, in partnership with the National Microbiology Laboratory from the Public Health Agency of Canada, have been collaborating on the Canadian Wastewater Survey (CWS) to monitor the levels of SARS-CoV-2, the virus that causes COVID-19, in the wastewater of five Canadian cities. As the virus continues to circulate, the CWS provides valuable insights on its activity in the population.
This release looks back at observed trends from January 2023 to February 2025.
SARS-CoV-2 wastewater levels were elevated in early 2023 but declined in the summer
SARS-CoV-2 levels were moderately high in all cities studied in the first half of 2023, following the arrival of a new Omicron sub-lineage in Canada in late 2022. Concentrations in Toronto, Montréal and Halifax were higher in the new year, then eased in the spring of 2023. In contrast, levels in Metro Vancouver and Edmonton were relatively low from January through February then rose until around May. Peak SARS-CoV-2 concentrations in Edmonton were not observed until early May, slightly after the end of flu season.
The summer months of 2023 were marked by relatively low levels of SARS-CoV-2 in wastewater. Levels declined near the start of June across all cities studied and remained low through September.
Increased activity during fall 2023 and winter 2024
SARS-CoV-2 levels began to increase around October 2023 and into the winter, possibly due to the arrival of another Omicron sub-lineage. Halifax and Metro Vancouver saw large increases in wastewater levels of the virus in October 2023. Apart from Metro Vancouver and Edmonton, concentrations of SARS-CoV-2 peaked during January 2024.
Fluctuations in summer 2024
After being elevated in early 2024, wastewater levels of the virus in Montréal and Toronto remained relatively low during the summer, while viral activity increased in Metro Vancouver and Halifax. This increase is unusual for respiratory illnesses, which typically decline in activity in summer. Levels in Edmonton during the summer of 2024 were slightly lower than winter levels, but they were higher than in the previous summer.
Fall 2024, winter 2025, onto the present
SARS-CoV-2 concentrations in Halifax were higher in fall 2024 compared with earlier in the year but remained relatively stable into 2025. Concentrations in Montréal followed a similar pattern but saw an increase in levels in early February 2025. From fall 2024 to February 2025, large fluctuations of increasing levels were observed in Edmonton and Toronto. Not following typical flu season patterns, levels in Metro Vancouver decreased from summer highs in the fall and winter.
Conclusion
Recent wastewater data indicate the absence of a strong seasonal pattern in SARS-CoV-2 concentrations over the past two years. Some cities have experienced fluctuations and elevated levels in warmer months. This is likely due to factors such as the virus' higher transmissibility compared with influenza, the influence of emerging sub-lineages, and waning immunity from vaccination and previous infection. Ongoing wastewater surveillance will shed light on the evolution of SARS-CoV-2 as it continues to circulate, which will support public health planning.
Infographic 1
Levels of SARS-CoV-2 virus in wastewater in Edmonton
Infographic 2
Levels of SARS-CoV-2 virus in wastewater in Halifax
Infographic 3
Levels of SARS-CoV-2 virus in wastewater in Metro Vancouver
Infographic 4
Levels of SARS-CoV-2 virus in wastewater in Montréal
Infographic 5
Levels of SARS-CoV-2 virus in wastewater in Toronto
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Note to readers
Wastewater analysis can complement other epidemiological indicators on the burden and prevalence of COVID-19, influenza (A and B), respiratory syncytial virus and mpox (previously called monkeypox). Wastewater analysis is timely, potentially cost-effective and easy to deploy, especially to remote areas where resources to carry on systematic clinical testing can be limited. Wastewater analysis continues to be an important indicator for SARS-CoV-2 surveillance and has been extended to the seasonal influenza virus (including both influenza A and B), respiratory syncytial virus and mpox (see Wastewater monitoring dashboard).
In early 2020, Statistics Canada initiated a collaboration with the Public Health Agency of Canada on a wastewater-based epidemiology program to detect and monitor levels of SARS-CoV-2 in the wastewater of five Canadian cities, representing approximately 9 million Canadians or 22.5% of the Canadian population. While this release focuses on these five Canadian cities, the Canadian Wastewater Survey added the following cities and regions to the joint collection program in 2024: Calgary (Alberta), Kingston (Ontario), London (Ontario), Moncton (New Brunswick), Peel Region (Ontario), Sudbury (Ontario) and Victoria (British Columbia). This collaboration is part of ongoing national and provincial programs of wastewater surveillance for infectious diseases used to complement and support existing surveillance programs and public health decision making.
Thresholds to define low, moderate and high levels are calculated considering all the available days that the virus was detected in at least one of the catchments of the city. High values are those above the 75th percentile, low values are those below the 25th percentile, and moderate values are in between those percentiles. The thresholds depend on the last season, when data are available, and the current season, and they will likely change as more data become available in the future. The plots display the viral concentration values representing the seven-day rolling mean.
Limitations
Despite the advantages listed above, there are some methodological and analytical limitations to the approach, meaning the results should be interpreted with caution.
Samples are only collected twice weekly, which, while maintaining accuracy regarding overall trends, can limit data interpretation and trend modelling in narrow time frames.
There are limitations associated with analytical aspects:
- Estimates of viral load are greatly affected by normalization techniques when using different indicators of the levels of fecal content in the wastewater. These data have not been normalized.
- Some variability has been observed when different fractions of the same wastewater sample (liquid versus solid) are repeatedly analyzed.
- External factors such as the weather can have important effects. Year-long differences in ambient temperature will affect how stable the virus genomic particles are in wastewater, impacting accuracy during warm months. Snowmelt, flooding, excess rainfall and drought conditions will contribute to dilution/concentration of viral signal.
Contact information
For more information, or to enquire about the concepts, methods or data quality of this release, contact us (toll-free 1-800-263-1136; 514-283-8300; infostats@statcan.gc.ca) or Media Relations (statcan.mediahotline-ligneinfomedias.statcan@statcan.gc.ca).
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