Why Canada is predicted to have a bad flu season and what vaccine mismatch has to do with it
A shift in the dominant strain of influenza could lessen the effectiveness of the annual vaccine and spell a rocky flu season for Canada, according to Dr. Isaac Bogoch, a professor at the University of Toronto in the Department of Medicine, and an infectious diseases specialist at the Toronto General Hospital.
There is, however, one surefire way to lessen the impact.
“People should get their flu shot,” he said bluntly in an interview with National Post. “That’s the take-home point.” He continued: “My concern is that people hear there’s a mismatch with the flu virus and say, ‘I don’t think I’m going to get one.'”
But the virus comes in three forms, he said — two strains of Influenza A known as H1N1 and H3N2, plus Influenza B. And the annual vaccine is designed to protect against all three.
“Even if there is a mismatch with one, you can protect yourself from the other two,” Bogoch said. “And even if there is a mismatch with one, you’re still provided with some protection. It just might not be the same degree of protection. So I’m very concerned about some of the messaging I’ve been seeing about this.”
Flu cases are on the rise in Canada, with government figures showing 1.9 per cent of tests coming back positive, and 431 detected cases in the week ending Nov. 1, in the latest published data. Similar figures were released by Ontario’s public health office this month.
Of those national cases, 98.6 per cent were influenza A, with H1N1 being the most commonly detected type . But medical experts say a mutation in the H3N2 strain could be a mismatch for this year’s vaccine, and an uptick in the number of cases of that strain around the world is concerning.
“H1N1 flu seasons, at a population level, tend to be less severe than H3N2 flu seasons,” said Bogoch. “Individuals can still have a rough course regardless of what influenza virus is circulating. But we just know, in general, the H3N2 seasons typically put more strain on our healthcare system. And other northern hemisphere countries are seeing a bit more H3N2 compared to H1N1 right now.”
The most recent data from the U.K. noted that while influenza levels were still fairly low, they were already increasing. “This is an unusually early start of the influenza season,” the report noted.
Ireland similarly noted an increase of 37 per cent in its most recent week of data, to 197 cases from 144 in the previous week. Hospitalizations more than doubled, to 53 cases from 22 the week before.
An even more troubling message came from Australia , where the flu season hits earlier in the year due to the southern hemisphere’s winter being the same time as our summer.
A report there noted that 2025 was the worst year on record for influenza, with around 1.5 per cent of people experiencing a notifiable flu infection – a 10.8 per cent increase on the previous year, which had itself been the record high.
“Cases continue to climb,” the report said, “with over 17,600 lab-confirmed cases … as of 20 October, more than double the 7,201 figures in October 2024.”
It’s worth noting that Canada has higher vaccination rates than Australia. In that country, only 60.5 per cent of seniors were vaccinated, the lowest number since 2020. In Canada, the vaccination rate among seniors was 73 per cent last year, although the overall rate was only 42 per cent, similar to the previous year’s 43 per cent.
Bogoch would like those numbers to be higher. “Because we don’t need a crystal ball to know that influenza is going to rip through Canada every late fall and winter, as it’s done for … years, and as it will continue to do.”
He added: “We know flu shots are available for people six months of age and older. And you know these are free, widely available vaccines that will a help reduce the risk of infection. And they do a remarkable job in reducing the risk of serious infection, such as hospitalization. That’s that.”
Influenza cases are climbing in Canada. Flu shots are widely available, and can help lower the chance of infection and hospitalization.Below are this year’s flu trends (arrow) vs. the past three years. pic.twitter.com/juqZxUFXzx
— Isaac Bogoch (@BogochIsaac) November 4, 2025
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