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
Anaesthesiology specialist
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.
