Two new analyses, one from France and one from China, suggest that seasonal influenza vaccination provided moderate protection during the early months of the 2025–26 flu season, despite the rapid spread of influenza A(H3N2) subclade K viruses, which differ from the strains anticipated during vaccine development for the current flu season.

Most effective among kids

To estimate vaccine effectiveness (VE) across France, researchers from the Hospices Civils de Lyon in Lyon, France, analyzed data from 24,267 patients tested through a national laboratory network and published their findings yesterday in Eurosurveillance. Among patients tested from November 2025 to January 2026, 5,451 (22.5%) were positive for influenza, and VE against confirmed influenza cases was 36.4%.

Effectiveness varied by age. VE was highest among children ages 0 to 17 years, at 57.2%, followed by adults ages 18 to 64 years at 45.1%. Among adults 65 years and older, VE was lower, peaking at 27.7%.

The data show that influenza activity began earlier and intensified faster this season than last season. The percentage of patients who tested positive for the flu in week 48 of the 2025–26 season (late November) was 12.0% (512 of 4,258), compared with 8.2% (538/6,525) in the 2024–25 season. By week 52 of 2025–26 (late December), the percentage of patients who tested positive had risen to 36.2% (1,702/4,701), compared with 35.8% (1,630/4,554) in the 2024–25 season.

The early weeks of the current flu season were dominated by the A(H1N1)pdm09 subclade D.3.1.1 strain. Starting in week 49, A(H3N2) subclade K strains became more common and continued to increase. Of the over 500 samples collected and analyzed through week 50, 54.2% were H1N1, and 44.9% were H3N2, with subclade K being the most detected strain. 

“Despite a partial vaccine mismatch this season, interim analyses indicate a statistically significant vaccine effectiveness across all age groups,” conclude the researchers. “In the context of sustained influenza circulation in Europe, reinforcing vaccination uptake in the coming weeks is strongly recommended.”

Similar levels of effectiveness in Beijing

A parallel study from Beijing, China, published in the same issue of Eurosurveillance, suggests similar results. Among 9,579 patients tested from September to December 2025, 1,942 (20.3%) tested positive for influenza, and almost all of those positive tests (1,904) were H3N2 strains. Sequencing of 316 samples found that 84.8% of the H3N2 strains belonged to subclade K.

Even though the 2025–26 vaccine didn’t contain antigens for subclade K, VE against confirmed cases was 41.3%. VE against H3N2 strains specifically was 39.9%. 

Similar to the French findings, effectiveness varied by age: VE reached 70.9% among children aged 5 years and younger but dropped to 25.3% among adults 60 years and older. For children aged 6 to 17 years, VE was 53.1%, and for adults aged 18 to 59 years, it was 25.3%. 

School-aged children made up an unusually large share of cases this season, note the researchers, likely because outbreaks spread easily in schools, and subclade K is highly transmissible.  

Both datasets suggest that the 2025–26 flu vaccine may help reduce severe cases across age-groups, even when distinct strains are circulating, and the vaccine doesn’t contain antigens for a dominant strain. “The overall evidence indicates that, despite antigenic drift and mismatch between the vaccine strain and circulating viruses, influenza vaccination remained effective,” write the Beijing-based researchers. 

Both research teams conclude that increased vaccine uptake remains an important strategy in helping protect against severe illness during the remainder of the 2025–26 flu season.