2025-26 Flu Vaccine Offers Moderate Protection Despite Strain Mismatch, Studies Show
Despite the emergence of a new influenza strain, preliminary data suggests the 2025-26 flu vaccine is providing a significant degree of protection against infection, particularly among children. New analyses from France and China, published in Eurosurveillance, indicate moderate vaccine effectiveness even as the influenza A(H3N2) subclade K virus rapidly spreads – a strain not fully accounted for in this year’s vaccine development.
Early Flu Season Intensifies, Strain Shifts
Influenza activity began earlier and intensified more quickly this season compared to the 2024-25 season. In France, the percentage of patients testing positive for the flu in late November (week 48 of the 2025-26 season) reached 12.0% (512 of 4,258), up from 8.2% (538/6,525) during the same period last year. By late December (week 52), positive cases climbed to 36.2% (1,702/4,701), slightly higher than the 35.8% (1,630/4,554) recorded in the previous season.
Initially, the A(H1N1)pdm09 subclade D.3.1.1 strain dominated the early weeks of the season. However, beginning in late November, A(H3N2) subclade K strains became increasingly prevalent, ultimately representing 44.9% of analyzed samples by week 50, alongside 54.2% of H1N1 cases.
French Study Highlights Age-Related Vaccine Effectiveness
Researchers from the Hospices Civils de Lyon in Lyon, France, analyzed data from 24,267 patients to estimate vaccine effectiveness (VE). Their findings, released on Thursday, revealed an overall VE against confirmed influenza cases of 36.4% between November 2025 and January 2026.
Notably, VE varied significantly by age group. The highest level of protection – 57.2% – was observed in children aged 0 to 17 years. Adults aged 18 to 64 years experienced a VE of 45.1%, while those 65 years and older saw a lower VE of 27.7%. “Despite a partial vaccine mismatch this season, interim analyses indicate a statistically significant vaccine effectiveness across all age groups,” researchers concluded. They strongly recommend reinforcing vaccination uptake in the coming weeks, given the sustained circulation of influenza in Europe.
Parallel Findings in Beijing Confirm Broad Protection
A parallel study conducted in Beijing, China, yielded similar results. Analyzing data from 9,579 patients tested between September and December 2025, researchers found that 20.3% (1,942) tested positive for influenza, with nearly all positive cases (1,904) attributed to the H3N2 strain. Sequencing of 316 samples revealed that 84.8% of the H3N2 strains belonged to subclade K.
Despite the 2025-26 vaccine lacking antigens for subclade K, the overall VE against confirmed cases reached 41.3%, and specifically against H3N2 strains, it was 39.9%. Age-related variations were again apparent: VE was highest among children aged 5 years and younger at 70.9%, but dropped to 25.3% among adults 60 years and older. VE for children aged 6 to 17 years was 53.1%, and 25.3% for adults aged 18 to 59 years. The researchers noted that school-aged children comprised a disproportionately large share of cases, likely due to the ease of transmission within school settings and the high transmissibility of subclade K.
Vaccination Remains Key Despite Antigenic Drift
Both research teams emphasize that the 2025-26 flu vaccine can still help reduce severe illness across all age groups, even when circulating strains differ from those included in the vaccine. “The overall evidence indicates that, despite antigenic drift and mismatch between the vaccine strain and circulating viruses, influenza vaccination remained effective,” the Beijing-based researchers wrote.
Ultimately, both studies underscore the continued importance of increased vaccine uptake as a crucial strategy for protecting against severe illness throughout the remainder of the 2025-26 flu season.
