Public health officials are closely monitoring an emerging mutation in this year’s dominant flu strain, raising early questions about the potential effectiveness of the current vaccine. Reports from the United Kingdom and British Columbia indicate a mismatch between the circulating strains and one component of the 2025-2026 influenza vaccine, though experts emphasize it is too early to definitively assess the impact on vaccine protection. The situation underscores the ongoing challenge of predicting and preparing for the ever-evolving influenza virus.
This year’s flu season is showing signs of an earlier-than-expected mutation, raising questions about the effectiveness of the current vaccine. Data from the United Kingdom and British Columbia indicate that one of the strains included in this year’s vaccine doesn’t fully match the strains currently circulating. However, experts caution that it’s too soon to determine whether the vaccine will be significantly less protective.
Why Might This Year’s Flu Vaccines Be Less Effective?
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The potential for reduced effectiveness stems from a mismatch between one of the three strains included in the vaccine and the strain currently spreading, according to Jesse Papenburg, an infectious disease specialist at the Montreal Children’s Hospital.
“It’s still too early to say if the vaccine will be less effective, but we know the H3N2 strain is different from what’s circulating,” said Dr. Papenburg, who also chairs the Quebec Immunization Committee. This mismatch is a key factor public health officials monitor each year as they assess vaccine efficacy.
Flu vaccines typically protect against three strains: two influenza A strains (H3N2 and H1N1) and one influenza B strain. Influenza B is often responsible for a second wave of flu cases later in the winter, Dr. Papenburg explained.
How Can You Protect Yourself Against This New Flu Strain?
Despite the potential mismatch, Dr. Papenburg emphasizes that vaccination remains the best defense.
You shouldn’t think the vaccine won’t protect against the flu this year.
Dr. Jesse Papenburg, infectious disease specialist at the Montreal Children’s Hospital
For those seeking enhanced protection, Benoît Mâsse, an epidemiologist at the University of Montreal, suggests asking for the high-dose vaccine, either Fluzone or Fluad. “Normally, this vaccine should be used for at-risk groups, such as those over 65,” Mâsse said. “But it’s worth asking for. I called a few pharmacies last week and they didn’t have Fluad.”
These enhanced vaccines aren’t free for those outside the at-risk groups, as they are significantly more expensive. “But if you’re 63, you can choose to pay, especially if you have insurance,” Mâsse added.
In the United Kingdom, the emergence of the new H3N2 strain has underscored the importance of a pilot program to vaccinate school children, Dr. Papenburg noted. “Often, the season starts when the virus circulates in schools and children infect adults,” he said. “Children don’t often require hospitalization, but their vaccinated immunity lasts longer. And if parents are told their child will be less likely to miss school, they may decide the vaccine is worthwhile.”
How is the Composition of Flu Vaccines Determined?
Public health specialists, including the World Health Organization (WHO), which publishes its recommendations, base vaccine composition on the previous year’s vaccines and the strains circulating in the Southern Hemisphere. Data from Australia is particularly important in this process.
“We only saw this H3N2 strain overtake the other strains late in the Southern Hemisphere’s season,” said Danuta M Skowronski, of the British Columbia Centre for Disease Control, who published a study on the topic in October in the Journal of the Canadian Medical Microbiology Association (JAMMI). Dr. Skowronski leads the Canadian Sentinel Surveillance Network, which collects data from medical clinics.
Have There Been Other Years When the Vaccine Was Poorly Matched to Circulating Flu Strains?
The 2014-2015 flu season was particularly challenging, with vaccine effectiveness dropping to less than 10%, compared to the usual 40-60%. Canada recorded over 550 deaths during that season, significantly higher than the typical 250-350.
“I think the vaccine will be more effective this year,” Dr. Skowronski said. “In 2014-2015, the H3N2 strain hadn’t changed from the previous year, but this year it has. The H3N2 strain in the vaccine is closer to the new strain. So, I believe there will still be some protection from the vaccine against the new H3N2 strain.”
3
Number of years in the past 20 where flu vaccine effectiveness exceeded 60%
4
Number of years in the past 20 where flu vaccine effectiveness was below 40%
Source: Canadian Sentinel Surveillance Network
