Flu cases in Canada reached a three-year high by the end of 2025, and while the numbers have recently stabilized, they remain high. Meanwhile, some hospitals across Canada have reported hospitalizations due to influenza have added to overcrowding issues.
Why is the flu hitting so hard this season? Jennifer Guthrie, Schulich School of Medicine & Dentistry professor and Canada Research Chair in Pathogen Genomics and Bioinformatics, explains the challenges that come with high rates of influenza and what people can do to protect themselves.
Western News: Why is the flu so rampant this year?
Jennifer Guthrie: There are a few reasons this flu season is worse than usual. Fewer people got vaccinated – including those most at risk – and there is reduced natural immunity due to milder flu seasons the last few years. Combine these with a slight vaccine mismatch to the dominant strain currently circulating, and we get a season where we see rates as high as we have now.
What caused the vaccine mismatch?
JG: The flu shot is made using strains that are circulating six to nine months in advance of flu season, to enable large-scale vaccine production. A flu vaccine mismatch happens when the flu viruses currently spreading have changed enough, genetically, from the strains used to make the vaccine.Therefore, the immune protection from the vaccine does not block infection as well as expected.
However, even in years with a mismatch, vaccination still helps reduce the risk of severe illness, complications, hospitalization and death because it better prepares the immune system to fight the virus, even if it does not fully prevent infection.
Is there a specific strain causing higher rates of infection?
JG: Yes, this year the Influenza A H3N2 strain is dominating, with a sub-variant called subclade K that has risen sharply in North America. H3N2 causes moderate to severe illness. Flu seasons dominated by this strain tend to have higher hospitalization rates and increased mortality.
Are there other viruses circulating as well?
JG: Other viruses, including COVID-19, RSV and Entero/Rhinovirus – which all have similar symptoms to influenza – are circulating at about the same rate as normally seen. Having any one of them can make someone more susceptible to getting another.
What are some of the concerns with a high rate of flu?
JG: When there is high flu activity, particularly with a strain that has increased transmissibility and causes more severe disease, it leads to more serious illness. A higher rate of serious infections will increase the use of health-care systems and increase the health risks for those most vulnerable. Older adults, very young children, pregnant people and individuals with chronic health conditions are more likely to experience flu complications.
Is it too late to get a flu shot?
JG: No, it is definitely not too late. The flu shot only takes about two weeks for the immune system to build full protection and there are still several months left of flu season.
What are some misconceptions about the flu shot?
JG: The two biggest misconceptions are that the flu shot can give you influenza, and that it is not effective. The vaccine uses inactivated virus or pieces of the virus, which cannot infect you, although you may feel achy and tired for a few days after being vaccinated as your body is working to build immunity.
The second belief – that the vaccine doesn’t work because some vaccinated people still get the flu – is also a myth. It’s important to remember the flu virus changes every year, and no vaccine can prevent every single infection. That said, getting vaccinated still makes a big difference. It greatly reduces the risk of severe illness, hospitalization and death, and it also helps limit the spread of the flu, which protects the people who are most vulnerable.
How can people protect themselves beyond the flu shot?
JG: There are two key recommendations for the best prevention. Firstly, hand washing frequently with soap and water for at least 20 seconds. If soap and water aren’t available, use an alcohol-based hand sanitizer. Secondly, consider wearing a well-fitted mask in crowded indoor spaces or if you’re around people at higher risk.
