2025-26 Flu Vaccine: Lower Effectiveness Against A(H3N2) Strain in Canada

Preliminary findings from a Canadian study evaluating the effectiveness of the 2025-2026 influenza vaccine against A(H3N2) virus infections are now available. The research, conducted between October 26, 2025, and January 10, 2026, across Alberta, British Columbia, Ontario, and Quebec, analysed data from individuals aged one year and older seeking medical care for acute respiratory infections.

Viral Landscape

Of the 4,873 samples analysed, 2,121 (44%) tested positive for influenza, with 2,092 (99%) identified as type A. A single case of co-infection with both influenza A and influenza B was also detected. Further analysis of the 1,959 subtyped influenza A samples revealed that 1,694 (86%) were A(H3N2), while 263 (13%) were A(H1N1)pdm09, with two instances of co-infection.

Did You Know? The study involved analyzing samples from patients experiencing acute respiratory infections for less than seven days.

A significant portion – 835 out of 1,696 (49%) – of the sequenced A(H3N2) viruses belonged to the 2a.3a.1 clade. Within this clade, 729 (87%) were classified as the K sub-clade, a descendant of J.2, differing from the 2025/26 vaccine strain. The remaining 106 (13%) belonged to the J.2 sub-clade or its descendants.

Vaccine Compatibility

Antigenic characterization revealed a concerning trend: 102 out of 112 (91%) viruses tested were “incompatible” with the current vaccine. Specifically, 79 of 79 viruses from the K sub-clade were incompatible, with 61 (77%) showing a 16-fold or greater reduction in hemagglutination inhibition assay (IHA) titers. However, most non-K variants (10/12) showed compatibility, with the exception of J.2.3 viruses (2/12).

In contrast, the vast majority of the 135 sequenced A(H1N1)pdm09 viruses (133) fell within the 5a.2a.1 clade, which aligns with the vaccine strain. Only one A(H1N1)pdm09 virus did not correspond to the vaccine.

Vaccine Effectiveness Estimates

The study assessed vaccine effectiveness (VE) among 4,448 participants – 1,696 cases and 2,752 controls. The median age of participants was 35 years, with cases of A(H3N2) tending to be younger (median age of 23 years) than cases of A(H1N1)pdm09 and the control group (median age of 41 years for both).

Expert Insight: The observed age distribution of A(H3N2) cases, with a younger median age, suggests potential influence from pre-existing immunity, a factor not as pronounced with the A(H1N1)pdm09 virus.

The overall VE against A(H3N2) infections requiring outpatient medical care was 40% (95% confidence interval: 28-49%). VE was 37% (95% CI: 20-50%) for sequenced viruses of the K sub-clade and 32% (95% CI: -25 to 63%) for viruses related to the J.2.4 sub-clade. VE varied by age group, ranging from 36% (95% CI: 10-55%) in those aged 1-17 to 25% (95% CI: -7 to 48%) in those over 65.

Notably, VE was highest among those aged 18-29 (63%, 95% CI: 32-80%) and 50-64 (56%, 95% CI: 29-73%), and lower among those aged 30-49 (30%, 95% CI: 0-51%). These preliminary VE estimates are lower than those reported in England and Europe, but higher than predictions based on virological assessments.

The study also found the VE against A(H1N1)pdm09 infections to be 31% (95% CI: 3-50%), though this finding is based on a small sample size.

Frequently Asked Questions

What types of influenza viruses were most prevalent in this study?

The study found that 86% of the influenza A viruses analysed were A(H3N2), while 13% were A(H1N1)pdm09.

How compatible were the circulating viruses with the 2025-2026 influenza vaccine?

A significant 91% of viruses characterized antigenically were found to be “incompatible” with the vaccine, particularly the K sub-clade of A(H3N2).

What was the overall vaccine effectiveness against A(H3N2) infections?

The overall vaccine effectiveness against A(H3N2) infections requiring outpatient medical care was estimated at 40% (95% confidence interval: 28-49%).

Should the current trends continue, public health officials may need to consider the potential for reduced vaccine effectiveness and adjust vaccination strategies accordingly. Further monitoring of viral evolution and ongoing assessments of vaccine performance will be crucial in the coming months. What impact might these findings have on future influenza vaccine development and strain selection?