Clinical and virological characteristics of critically ill patients with influenza in France during the 2025/26 season, marked by the emergence of influenza A(H3N2) clade K

ImportanceRecent reports have highlighted an intense influenza activity related to the circulation of the influenza A(H3N2) subclade k variant. There is no data available on the impact of the emergence of H3N2 subclade k on the severity of the 2025-2026 epidemic or on the clinical phenotype of patients requiring admission to the intensive care unit (ICU). ObjectiveTo compare the clinical presentation, hospital mortality and virological characteristics of patients with laboratory-confirmed influenza infection included in French intensive care units during the 2025-2026 epidemic season with those of patients admitted during the 2024-2025 season. We also aimed at measuring the impact of the A(H3N2) subtype on hospital mortality during the 2025-2026 season. DesignProspective, multicenter, observational SEVARVIR cohort study including patients admitted during the 2024-2025 and 2025-2025 influenza seasons. SettingForty-two French ICUs ParticipantsAdult patients with laboratory-confirmed influenza infection Interventionsnone Main Outcomes and MeasuresThe primary outcome measure was in-hospital mortality. ResultsPatients admitted in intensive care units for influenza in 2024-2025 (n=360) and 2025-2026 (n=325) were included in the French nationwide prospective multicentre SEVARVIR study. There was no significant difference in day-28 mortality between the seasons (12.7%, n=45/355 vs 16.5% n=28/170; p=0.28). In the 2025-26 season, 49% had the A(H1N1) subtype and 51% the A(H3N2) subtype (k subclade: 77%). The univariable Cox analysis revealed that patients infected with A(H3N2) viruses were at greater risk of death over time. Multivariable Cox analysis revealed that during the 2025-2026 season, age (adjusted hazard ratio, aHR=1.05 [1.00;1.11]; p=0.046) and the clinical frailty scale (aHR=1.82 [1.26;2.72]; p=0.001) were associated with an increased risk of death. The A(H3N2) subtype was not associated with an increased risk of death (aHR=1.13 [0.32;4.51]; p=0.85). Phylogenetic analyses from our ICU cohort together with 300 contextual sequences from community-acquired influenza cases collected during the same period showed no clustering according to severity. Conclusions and RelevanceThis French national prospective observational study, found that the emergence of the influenza A(H3N2) subclade K was associated with an increased risk of death in univariable but not multivariable analysis, adjusting for host-related factors. Trial RegistrationNCT051625 Key PointsQuestion: What impact did the 2025-26 influenza epidemic and the A(H3N2) variant have on the mortality of patients admitted to intensive care units? Findings: In this prospective, nationwide cohort study of 685 patients admitted to intensive care units with severe influenza during the 2024-25 or 2025-26 seasons, no difference in hospital mortality was observed between the two seasons. Patients infected with the A(H3N2) virus, 77% of which corresponded to clade k, were at higher risk of death in univariable but not in multivariable analysis after adjusting for age and clinical frailty scale. Meaning: Patients in intensive care units with severe A(H3N2) infection during the 2025/2026 season were not at higher risk of death after adjusting for confounding variables.