Rates of influenza A are far higher than last winter in Toronto, sewage testing data shows.
In early January, influenza A spiked suddenly, quickly reaching rates more than double the maximum levels from last winter. The presence of influenza A in the city’s wastewater has since fallen, but is still higher than the 2024 peak.
The Ashbridge's Bay wastewater treatment plant, where the samples were taken, handles sewage from Toronto's downtown, and from the east and west ends of the older part of the city.
The data below is current to the middle of February.
The sewage-based data is consistent with reports from hospitals and with Health Canada's weekly influenza report.
Health Canada reported that test positivity — the percentage of tests that come back positive — for influenza is at its highest rate since the start of the 2020-21 season, and that "some continued increase is possible."
It also said hospitalizations for flu have increased, especially for those over 65. Since last August, flu has accounted for 205 ICU admissions and 104 deaths in Canada.
People infected with influenza may experience fever, coughs and muscle aches. They may also have headaches, chills and fatigue.
The rate of respiratory syncytial virus (RSV) has also risen this winter, but to more seasonally normal levels. Most people experience RSV as a cold, but it can be more dangerous for babies, the elderly and people who are immunocompromised.
Influenza B spiked in the winter of 2023-4, but not this winter to the same extent.
Systematic testing of sewage for diseases began during the COVID-19 pandemic.
It offers an alternative to relying on medical data, like positive lab tests, for tracking disease in a community.
Not every person with a given disease ends up tested for it, or experiences symptoms serious enough that they seek medical attention. Sewage, however, offers the possibility of a data source that everybody — young or old, rich or poor — participates in creating.
As a data source, sewage can also provide earlier signals than test results. Often, people are sick for a few days before seeking medical attention, and there can be further delays before a positive test result ends up in the statistics.
In New Haven, Ct. in April of 2020, for example, samples from local sewers showed COVID trends a full week before they showed up in public health statistics based on tests.
Controversially, Queen's Park shut down the province's wastewater testing unit last June. It previously hosted 58 testing locations spread out across the province's regions, including 18 in northern Ontario.
Now, a much smaller federal program tests at seven Ontario sites: four in Toronto, two in London and one in Mississauga.
