Here’s a startling fact: even when the flu vaccine doesn’t perfectly match the circulating strains, it can still offer significant protection against severe illness. But here’s where it gets controversial—how effective is it really, especially when the dominant strain isn’t included in the vaccine? Two recent studies, one from France and another from Beijing, shed light on this critical question, and their findings might surprise you.
During the early months of the 2025–26 flu season, researchers discovered that the seasonal influenza vaccine provided moderate protection, even as the highly contagious influenza A(H3N2) subclade K viruses spread rapidly. These strains were not the ones anticipated during the vaccine’s development, yet the vaccine still proved valuable. And this is the part most people miss—it wasn’t just effective; it was most effective among children, with protection rates as high as 70.9% in kids aged 5 and younger in Beijing and 57.2% in French children up to 17 years old.
In France, researchers from the Hospices Civils de Lyon analyzed data from 24,267 patients tested through a national network. Among them, 22.5% tested positive for influenza, and the vaccine effectiveness (VE) against confirmed cases was 36.4%. The protection varied by age: adults aged 18–64 saw a 45.1% effectiveness rate, while seniors 65 and older had a lower but still notable 27.7%. What’s striking is how early and aggressively the flu hit this season. By late December 2025, 36.2% of tested patients were positive, compared to 35.8% during the same period in 2024–25. The A(H3N2) subclade K strains became dominant starting in late November, overtaking the earlier A(H1N1)pdm09 subclade D.3.1.1 strain.
Despite this mismatch, the French researchers concluded, “Interim analyses indicate a statistically significant vaccine effectiveness across all age groups.” They strongly recommend boosting vaccination rates to curb the ongoing flu circulation in Europe.
Meanwhile, in Beijing, a parallel study of 9,579 patients found similar results. Among the 20.3% who tested positive for influenza, nearly all cases were H3N2 strains, with 84.8% belonging to subclade K. Even though the 2025–26 vaccine lacked antigens for this subclade, it still provided 41.3% effectiveness against confirmed cases and 39.9% against H3N2 specifically. Again, children fared best, with 70.9% protection in those 5 and younger, while adults 60 and older saw just 25.3%.
Here’s the bold takeaway: School-aged children made up an unusually large share of cases this season, likely due to the highly transmissible nature of subclade K and the ease of outbreaks in schools. Yet, the vaccine still helped reduce severe cases across age groups, even without matching the dominant strain. “Influenza vaccination remained effective despite antigenic drift,” the Beijing researchers noted.
Both studies emphasize that increasing vaccination uptake is crucial to protecting against severe illness for the rest of the flu season. But this raises a thought-provoking question: If the vaccine can still offer protection despite mismatches, should we rethink how we approach flu vaccine development and distribution? Let us know your thoughts in the comments—do you think we’re doing enough to maximize the vaccine’s potential, or is there room for improvement?
