Emerging influenza threat: the rapid rise of A(H3N2) subclade k in Europe

By Carolina Lima | Published on Décember 30, 2025 | 3 min read


The European Centre for Disease Prevention and Control (ECDC) published a Threat Assessment Brief on 20 November 2025 addressing the growing circulation of influenza A(H3N2) subclade K across Europe. This K variant has spread quickly across Europe and other regions over the past months accounting for a significant proportion of influenza detections between May and November, 2025.

Influenza remains a major seasonal health burden in the EU/EEA, with up to 50 million symptomatic cases annually and 15,000–70,000 deaths. The emergence of subclade K raises concerns about increased transmission, potential vaccine mismatch, and added strain on healthcare systems.

In France, Santé publique France reported in its national bulletin of 10 December 2025 that nearly all metropolitan regions had entered the epidemic phase, with subclade K identified as the dominant strain among circulating influenza A(H3N2) viruses. The bulletin highlighted a rapid rise in consultations and hospital admissions across all age groups, underscoring the early and intense nature of this season’s epidemic. These findings reinforce the ECDC’s warning that subclade K may significantly amplify morbidity, mortality, and healthcare system pressure during the 2025–2026 influenza season.

Epidemiological situation

• Rapid spread: Subclade K has expanded quickly in Europe and East Asia, representing nearly half of H3N2 sequences reported globally in late 2025.
• Age distribution: All age groups are affected, but children show higher infection rates and often act as primary transmitters within households and community transmission.
• Seasonal burden: Up to 20% of the population contracts influenza each year and early signals suggest subclade K may precipitate more intense outbreaks earlier in the season.

Risk assessment

Morbidity: High attack rates are expected among children and vulnerable adults. Genetic changes in the K subclade of A(H3N2) may enhance transmissibility, potentially triggering earlier and more extensive outbreaks compared to previous seasons.

Mortality: seasonal influenza already results in tens of thousands of deaths annually in Europe. The ECDC warns that if subclade K spreads widely, mortality could rise, particularly among older adults, those with chronic conditions, and immunocompromised individuals.

Healthcare Impact: Concurrent circulation of influenza, SARS‑CoV‑2, and RSV poses a compounded threat. Increased admissions, ICU demand and staff absenteeism are expected, potentially reducing system resilience.

Vaccine Effectiveness Concerns: ECDC warns of a possible antigenic mismatch between current vaccine strains and the K subclade, which may reduce vaccine effectiveness. Despite this, vaccination remains critical: it provides partial protection, mitigates disease severity, and lowers hospitalization risk. Continuous monitoring of vaccine performance is essential, with potential strain updates for the 2026 season under consideration.

Recommendations

The ECDC outlines several key actions for EU/EEA countries:

• Enhanced surveillance through molecular and epidemiological monitoring to assess spread and impact.
• Promote vaccination campaigns to maximize coverage, emphasizing benefits even in the context of reduced effectiveness.
• Strengthen healthcare preparedness by ensuring adequate hospital capacity and contingency planning for workforce shortages.
• Communicate effectively with the public and healthcare professionals regarding preventive measures and the importance of early medical consultation for high‑risk groups.
• International collaboration: Share data across countries to improve situational awareness and guide vaccine strain selection.

Overall conclusion

The ECDC identifies the growing dominance of A(H3N2) subclade K as a moderate to high risk for the 2025–2026 influenza season. This variant’s rapid spread, combined with potential vaccine mismatch, raises concerns about increased transmission, higher morbidity and mortality, and significant strain on healthcare systems—especially amid concurrent circulation of SARS-CoV-2 and RSV. Vaccination remains the most effective tool to reduce severe outcomes, even with partial effectiveness, and should be reinforced through widespread campaigns. Complementary measures, including enhanced molecular surveillance, international data sharing, and robust preparedness planning, are essential to mitigate the impact and guide future vaccine strain selection.

Read next: Flu at school: what if it all began with education?

About the Author – Carolina Lima

Carolina is a specialist in Anaesthesiology with a deep passion for learning and sharing medical knowledge. Dedicated to advancing the field, Dr. Lima strives to bring fresh, evidence-based insights to the medical community. Viewing medicine not merely as a profession but as a lifelong journey of continuous learning, Dr. Lima is committed to making complex information clear, practical, and useful for healthcare professionals around the world.