BOSTON — This flu season's dominant influenza strain is partially slipping past existing immunity, according to a new laboratory study, even as health officials in several countries urge people not to wait to get vaccinated. The findings do not mean vaccination is useless — researchers say the shot still boosts protection — but they help explain why a fast-moving early season is being watched so closely on both sides of the Atlantic.
The study, led by Beth Israel Deaconess Medical Center (BIDMC) and published as a research letter in the journal JAMA, focuses on influenza A H3N2 "subclade K," the variant now accounting for more than 88 percent of influenza detections in surveillance reports cited by the researchers. Its message is nuanced: immunity built up from earlier flu strains, and from this season's vaccine, recognises subclade K less well than it recognises older viruses — yet vaccination still raises antibody levels against it.
Illustration: A stylised influenza virus particle under laboratory study, representing the H3N2 subclade K variant.
What Happened
Researchers at BIDMC compared the hemagglutinin protein — the main surface protein that antibodies recognise — on subclade K with the H3N2 protein in this season's vaccine. They found 11 changes in that protein, including eight at sites where antibodies usually attach. That, they say, helps explain why subclade K can partly evade immune recognition.
The team then measured antibody responses in 46 healthy adults in Boston who received the 2025 trivalent inactivated influenza vaccine. Blood samples taken before vaccination and three weeks later were tested against five influenza strains: two subclade K viruses from 2025 and three earlier H1N1 and H3N2 strains.
Participants mounted strong antibody responses to the non-K strains but weaker responses to subclade K, the study found. People started with lower antibody levels against subclade K, and after vaccination they produced smaller increases than against the other strains. Following vaccination, participants had three- to eight-fold lower neutralising antibodies against the K strains than against the non-K strains; another standard laboratory test, the hemagglutination-inhibition (HAI) test, showed the same pattern.
"The rapid expansion of H3N2 subclade K is a major public health concern because both baseline and vaccine-induced immunity to this variant is lower than expected," corresponding author Dan H. Barouch said in BIDMC's report on the study. First author Liping Wang added that the vaccine "still provides meaningful benefit," but that subclade K "looks different enough from the strains the vaccine was designed to match that the boost in antibodies is more modest."
That distinction matters. A lower antibody boost in the laboratory is not the same as no protection, and laboratory antibody levels are only one part of how the immune system defends against severe illness. But the result does point to a mismatch driven by viral evolution, known as influenza drift, at a time when the season is already under way.
Background
Influenza viruses evolve every year, and small changes in their surface proteins can make them look different to the immune system, weakening the protection from prior infection or vaccination. Subclade K is the latest example: a branch of H3N2 that has spread rapidly while older strains circulate at lower levels, with H1N1 reported at lower levels in the surveillance cited by the BIDMC team.
The same pattern — a fast-moving early season — is showing up in regional reports. In Ontario, Canada, public health data reported on November 3, 2026, show influenza cases gradually rising week by week: 360 positive cases of all influenza types were reported across the province between October 18 and October 24, up from 305 the week before and 225 the week before that. The province's flu season began in early September, with free flu shots rolled out for some groups in late September and opened Ontario-wide on October 26.
Importantly, the Ontario picture also carries a more reassuring signal. Infectious diseases specialist Dr. Isaac Bogoch told CTV News Toronto that early testing by Canada's National Microbiology Laboratory, covering 116 samples, showed the current flu vaccine is a good match for the circulating influenza strains. "A pretty good match, like a really good match for the circulating flu," he said in the November 3 interview. Toronto Public Health said the same day that it was encouraging residents to get both flu and COVID-19 vaccines, noting 258 lab-confirmed COVID-19 cases in Ontario in the week ending October 24, and announced a vaccination clinic on November 5 at North York Civic Centre offering flu, COVID-19 and RSV vaccines to everyone aged six months and older.
Illustration: A person receiving a flu vaccination at a community clinic during the autumn vaccination campaign.
In the United States, the early-season message is similar: get vaccinated now rather than waiting for cases to rise. In Maryland, where flu activity remains relatively low so far, experts at the Johns Hopkins Bloomberg School of Public Health warned on November 3 that conditions could change quickly as the season progresses. Professor Andrew Pekosz noted that Maryland has not seen the same surge reported in some southern states, while Professor Anna Durbin stressed getting vaccinated early, especially as the season begins ahead of schedule in several regions. Because it takes about two weeks for the body to develop protection after vaccination, officials say, delaying the shot risks being unprotected when activity climbs.
Supply, however, is becoming a second front in the story. In England, community pharmacies reported on November 3 that flu vaccine stock is under pressure after strong demand in the first month of the programme. Community Pharmacy England said some pharmacy owners had used up all their stock of IIVc, the cell-cultured inactivated influenza vaccine that is the main first-line vaccine for the 18-to-49 age group, and that IIVc was no longer available to order from the manufacturer or wholesalers. The UK Health Security Agency had warned last month that England could face delays or more limited volumes of certain jabs this year, explaining in a patient leaflet that all three flu vaccines for this year's programme needed to be updated from last year's — an unusual situation — and that supply was therefore being manufactured and distributed "just in time" this autumn. Where the recommended vaccine has run out, patients should be directed to alternative providers with stock or told to rebook, under the vaccination service specification, with the egg-cultured IIVe vaccine as the alternative for the 18-to-49 group.
Why It Matters
Subclade K matters for three reasons. First, dominance: when one variant accounts for nearly nine in ten detections, even a modest reduction in immune recognition affects a large share of infections. Second, timing: the season started early in several places, so a partially evasive strain is circulating while vaccination campaigns are still ramping up. Third, perception: headlines about a strain "dodging" immunity can discourage vaccination, even though the study itself says the vaccine still boosts protection — the opposite of the conclusion health officials want people to draw.
Public-health experts quoted in the November 3 reports return to the same practical point: vaccination remains one of the most effective ways to reduce the risk of severe illness and complications, and it is most useful when given before activity peaks. The BIDMC findings add urgency to that advice rather than undermining it, because a population with lower baseline immunity to the dominant strain has less cushion if cases rise sharply.
The supply pressures in England show the other side of that urgency. If demand surges and a first-line vaccine runs out, people may face delays or need an alternative product — a manageable problem if communicated early, but a risk to confidence if people are turned away without a clear next step. Health systems are therefore juggling two tasks at once: explaining a complicated laboratory finding honestly, and making sure the shot itself is available when people act on the advice.
Illustration: A hospital respiratory ward during winter flu season, with medical staff caring for patients.
What Happens Next
Surveillance will be the first signal of what subclade K means in practice. Health agencies will watch weekly case counts, hospital admissions and the share of detections made up by H3N2 subclade K, alongside vaccine-effectiveness estimates that measure real-world protection against illness and hospitalisation — a broader test than laboratory antibody levels alone. Ontario's week-by-week rise, from 225 to 305 to 360 cases, is the kind of trend officials will track to judge whether the early season is building into a larger wave.
Vaccination campaigns will continue through the autumn, with clinics such as Toronto's November 5 session offering flu, COVID-19 and RSV vaccines together, and US experts urging people not to wait for a local surge. In England, pharmacies and wholesalers will be watched for whether alternative vaccine supplies ease the IIVc shortage, and whether patients who cannot get their first-choice jab are successfully redirected rather than lost from the programme.
Researchers, meanwhile, will keep tracking influenza evolution. The BIDMC study's authors note that subclade K's 11 hemagglutinin changes, eight of them at antibody-binding sites, are a reminder that each season's vaccine is designed against a moving target. Future strain selection will take account of how widely subclade K spreads and how well current vaccines perform against it in the real world — evidence that will only become clear as the season unfolds.
For now, the advice from the sources in this story is consistent and simple: the dominant strain is harder for the immune system to recognise, the vaccine still helps, protection takes about two weeks to build, and the best time to get it is before cases climb in your area.
This article reports public-health announcements and research findings. It is general information, not medical advice; anyone concerned about exposure or vaccination should contact a health care provider or local health department.
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FAQ
What is H3N2 subclade K?
It is a branch of influenza A H3N2 that has become the dominant circulating flu strain this season, accounting for more than 88 percent of influenza detections in surveillance reports cited by the BIDMC study.
Does the flu vaccine still work against subclade K?
According to the BIDMC study, yes — vaccination still boosts antibody protection against subclade K, but the boost is more modest than against earlier strains, with three- to eight-fold lower neutralising antibodies than against non-K strains.
Why does subclade K partly evade immunity?
Researchers found 11 changes in its hemagglutinin surface protein compared with the vaccine strain, including eight at sites where antibodies usually attach, making the virus look different to the immune system.
How fast is the flu season rising?
In Ontario, cases rose from 225 to 305 to 360 positive cases over three consecutive weeks to October 24, while US experts report an early start in some southern states and low but closely watched activity in Maryland.
Why is there a flu vaccine shortage in England?
Strong early demand plus an unusual year in which all three flu vaccines needed updating meant supply was manufactured "just in time"; the main first-line vaccine for ages 18 to 49, IIVc, is reported as no longer available to order, with IIVe as the alternative.
Sources
Beth Israel Deaconess Medical Center, "Influenza Strain Partially Dodges Existing Immunity and Current Vaccine, BIDMC-Led Study Finds," research letter in JAMA, reported November 2026. CTV News Toronto via EuropeSays, "Ontario flu season: how good is the current shot?," November 3, 2026. The Pharmacist, "Contractors run out of flu vaccine amid stock pressures," November 3, 2026. 92Q / Johns Hopkins Bloomberg School of Public Health, "Maryland Flu Cases Remain Low as Experts Urge Vaccinations," November 3, 2026.
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