While this fall’s influenza vaccine was a mismatch for the dominant strain, it still cut the risk of serious illness by roughly 40 per cent, according to a Canadian network that has long tracked the performance of the annual shot.
The Canadian Sentinel Practitioner Surveillance Network published its mid-season estimate on flu vaccine effectiveness in the Eurosurveillance journal on Thursday. The network is composed of hundreds of primary care providers in Alberta, B.C., Ontario and Quebec.
A mutated variant of influenza A (H3N2), called Subclade K, has caused the majority of infections this season. For months, infectious disease experts have warned that the strain may be an imperfect match to the current vaccine, which may cause it to offer less protection.
Flu numbers start to ease after hitting three-year peak in December
The network estimated the vaccine reduced the risk of serious flu infection by about 40 per cent overall compared with unvaccinated individuals, and specifically 37 per cent against Subclade K. This puts effectiveness in the “low mid-range” compared with the last eight mid-season estimates.
Danuta Skowronski, who leads the research network, said there were fears that the vaccine would be a more significant mismatch for the circulating strain. But the assessment was reassuring, she said, in that it revealed vaccination still offered worthwhile protection.
Reducing the risk of severe illness by 40 per cent is “pretty decent” by H3N2 standards, she said.
“Vaccine effectiveness, historically, has been lower against H3N2 compared to, say, Influenza B or H1N1,” explained Dr. Skowronski, who is a physician epidemiologist with the BC Centre for Disease Control. “So, that’s a very meaningful amount of reduction in risk, especially for those who are at high risk of severe outcomes.”
Another interesting finding by the network was that there were indications of pre-existing protection among unvaccinated adults, likely due to immunity accumulated through past infections. This may explain why children were more vulnerable to the Subclade K strain.
Nasal specimens were collected in Canada’s four largest provinces from patients who presented with acute respiratory illness between October, 2025, and January, 2026. About 44 per cent of the 4,875 specimens tested positive for influenza – mostly the H3N2 subtype.
Using this information, investigators estimated vaccine effectiveness, which is a measure of how much the risk of serious outcomes is reduced in vaccinated versus unvaccinated participants.
This year’s flu season hit hard and fast, with cases peaking at the end of December with a positivity rate of 33.3 per cent – the highest rate recorded over the past three viral seasons. The spread of infection has since slowed significantly and continues to dwindle.
Data from the Public Health Agency of Canada, updated on Friday, show the positivity rate dropped to 7.5 per cent during the week ending Jan. 31, down from 8.2 per cent the previous week.
I’ve heard the flu shot isn’t a good match for the virus this year. Should I still get it?
Canada offered a trivalent vaccine this season, which protects against two strains of Influenza A and one strain of Influenza B. But uptake has been lower in certain parts of Canada, which experts say could be a symptom of concerns that the vaccine was not a perfect match for the circulating strain.
In Alberta, for example, roughly 20 per cent of the population has been vaccinated against the flu so far this season. The 10-year average is nearly 28 per cent, with a high of 37 per cent during the 2020-21 season, which coincided with the height of the COVID-19 pandemic.
It’s a similar situation in Manitoba, where nearly 22 per cent of the population has been vaccinated to date this season, roughly three percentage points lower than the 10-year average. This province also saw a spike in coverage during the pandemic.
But immunization rates vary between provinces and territories. For example, coverage sits around 27 per cent in Nova Scotia, 25 per cent in B.C., 21 per cent in Saskatchewan and 19.5 per cent in the Northwest Territories, according to data provided to The Globe and Mail and through public surveillance.
Dr. Skowronski said people should still get the flu shot, but stressed that it’s best to get vaccinated early in the season for maximum benefit. She said the epidemic of H3N2 appears to have subsided, but she is now starting to see an increase in Influenza B, which disproportionately impacts children.
“The season is not over,” Dr. Skowronski warned. “So, it’s not too late to get the vaccine.”