Extended wastewater monitoring to influenza viruses in Canadian cities, August 27, 2023 to June 9, 2024
Released: 2024-07-22
Influenza (the flu) causes significant societal and healthcare burdens: it can rapidly spread and cause severe illness, exacerbate underlying medical conditions, and lead to hospitalization and death, especially among the youngest and oldest populations (see Note to readers).
Since 2020, Statistics Canada and the National Microbiology Laboratory from the Public Health Agency of Canada have been collaborating on the Canadian Wastewater Survey to monitor the spread of SARS-CoV-2 in participating cities across Canada. Testing for influenza and the respiratory syncytial virus (RSV) were added in 2022 to monitor trends and provide insights into recent flu seasons in selected cities. This includes both influenza A and B, the main influenza viruses that cause the seasonal outbreaks in humans.
The influenza season is a period with elevated influenza activity. The start of influenza season is declared when the percentage of positive laboratory tests on individuals potentially affected by influenza is above 5%. According to the Public Health Agency of Canada (see Note to readers), the current flu season started nationwide on the last week of November 2023, a bit ahead of the previous season.
The results of the wastewater analysis show that while all participating cities observed elevated flu levels at some point over the course of the winter of 2023-2024, they did not occur at the same time across cities or for the same duration. Flu A and flu B started circulating at sustained high levels earlier in the West of the country, first in Edmonton, and later moved East to Montréal and Halifax. Levels of flu A were consistently high although variable for longer stretches of time in Metro Vancouver and Toronto than in the other cities.
Metro Vancouver
Flu season began the last week of November 2023 in Metro Vancouver, British Columbia, when wastewater levels reached a level considered high (see Note to readers). Flu A levels in the wastewater of Metro Vancouver continued to climb, peaking the last week of December 2023, then generally remained high until the end of February 2024 and moderate from March onwards. During early February, flu B started circulating at higher levels in wastewater and remained high until the end of May before returning to low levels by early June.
Infographic 1
Levels of flu viruses A and B in wastewater in Metro Vancouver
Edmonton
The 2023-2024 flu season began the earliest in Alberta, during the first week of November 2023, and is so far reported as being the most severe in the province in the last five years, according to the Government of Alberta. In Edmonton, Flu A levels were high from mid-November 2023 to mid-January 2024. Flu B viral signals emerged in the wastewater of Edmonton during the second week of December 2023; levels were moderate until early February 2024, then peaked in early March before declining in April and May. Data were not consistently collected at the end of December and early January.
Infographic 2
Levels of flu viruses A and B in wastewater in Edmonton
Toronto
Overall, according to Toronto Public Health, Toronto saw an average to below average number of confirmed flu outbreaks during the 2023-2024 flu season compared to historical averages. Flu A levels in the wastewater of Toronto rose in early November 2023 when, according to Toronto Public Health, the number of reported cases of influenza jumped from 19 to 50. Wastewater flu A levels peaked in Toronto in December just before the holiday season and remained high from late November 2023 to early March 2024. Flu B levels in wastewater rose quickly at the beginning of February and remained high until the end of April.
Infographic 3
Levels of flu viruses A and B in wastewater in Toronto
Montréal
Flu A started circulating in wastewater in Montréal at moderate levels at the end of November 2023. During the December holiday season levels rose to high, peaking first in mid-January 2024 and again in mid-February. Levels remained high until the end of February. Flu B wastewater levels in Montréal started to rise in early February and fluctuated between high and moderate until the end of May. Data were not consistently collected during December 2023.
Infographic 4
Levels of flu viruses A and B in wastewater in Montréal
Halifax
Wastewater flu A levels in Halifax were low until January 2024. They were high and variable until March. Like in Montréal, flu A in wastewater peaked later than in the other cities. Flu B wastewater levels rose in February and fluctuated between moderate and high since then. Flu B levels in wastewater were highest in April and peaked again in the second half of May and dropped to low in June.
Infographic 5
Levels of flu viruses A and B in wastewater in Halifax
Flu A and flu B started circulating first in the West of the country while patterns of duration of high viral levels were also very different between the cities. More research is needed to precisely explain how these patterns and the differences between cities are related to infections in the population, considering wastewater monitoring strengths and limitations. However, several years of coupled wastewater and clinical data support that the trends are very similar for COVID-19, flu, and RSV, indicating that wastewater analysis can be an early warning sign and can support our understanding of the spread of infectious diseases by complementing other epidemiological indicators on the burden of flu such as number of hospitalizations or deaths.
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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), and respiratory syncytial virus. 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, and to respiratory syncytial virus (see Respiratory viruses: 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. 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.
Operating year-round to track flu activity across the country, FluWatch compiles data from multiple sources, including data from laboratory tests, hospitalizations and volunteers in the community. Its aim is to detect trends, identify circulating types, and assess the severity of flu outbreaks. By analyzing this data in near real-time, FluWatch enables early detection of flu epidemics, informs public health interventions, and guides vaccination strategies.
Thresholds to define low, moderate, and high levels are calculated considering all the available days where the virus was detected in at least one of the catchments of the city. High values are those above the 75th percentile, low values below the 25th percentile, and moderate in between those percentiles. The thresholds depend on the last season, when data are available, as well as the current season, and will likely change as more data becomes 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. This data has 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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