People With Influenza Requiring ICU Admission During the 2025-2026 Season, Marked by the Emergence of Influenza A(H3N2) Subclade K: A French Prospective Nationwide Cohort Study.
de Prost Nicolas N, Bay Pierre P, Le Goff Marie M, Préau Sébastien S et al.
Intense influenza activity has been reported alongside the emergence of influenza A(H3N2) subclade K. While epidemiological studies have reported heterogeneous findings regarding public health impact, data remain lacking on its impact on clinical phenotype of people requiring intensive care unit (ICU) admission. Prospective, multicenter, observational SEVARVIR cohort study including people admitted in 42 French ICUs with laboratory-confirmed influenza infection during the 2024-2025 and 2025-2026 seasons. Primary end point was day-28 mortality. Seven hundred twenty-six people were admitted for influenza in 2024-2025 (n = 360) and 2025-2026 (n = 366). In 2025-2026, influenza subtype was available for 265 people; 49% (n = 129) had A(H1N1)pdm09 and 51% (n = 136) had A(H3N2) subtype infection. Among the 40 people with A(H3N2) infection for whom whole-genome sequencing was available, 33 (82.5%) belonged to subclade K. In multivariable Cox analysis of the 2025-2026 season, age (adjusted hazard ratio, aHR = 1.04 [1.01;1.09]; P = .015), Clinical Frailty Scale (aHR = 1.59 [1.28;1.98]; P < .001), and sequential organ failure assessment score (aHR = 1.15 [1.05;1.26]; P = .003) were associated with an increased risk of death. In the 2024-2025/2025-2026 seasons or 2025-2026 season alone, influenza subtype was not associated with a significant risk of day-28 mortality. Phylogenetic analyses from our ICU cohort together with 600 contextual sequences from community-acquired influenza cases collected during the period showed no clustering according to severity. There was no evidence of an increased risk of death among critically ill people with influenza during the 2025-2026 season, despite the emergence of influenza A(H3N2) subclade K. NCT05162508.