Interim 2022/23 influenza vaccine effectiveness: six European studies, October 2022 to January 2023

Background: Between October 2022 and January 2023, influenza A(H1N1)pdm09, A(H3N2) and B/Victoria viruses circulated in Europe with different influenza (sub)types dominating in different areas. Aim: To provide interim 2022/23 influenza vaccine effectiveness (VE) estimates from six European stud-ies, covering 16 countries in primary care, emergency care and hospital inpatient settings. Methods: All studies used the test-negative design, but with differences in other study characteristics, such as data sources, patient selection, case definitions and included age groups. Overall and influenza (sub)type-specific VE was estimated for each study using logistic regression adjusted for potential confounders.Results: There were 20,477 influenza cases recruited across the six studies, of which 16,589 (81%) were influ-enza A. Among all ages and settings, VE against influ-enza A ranged from 27 to 44%. Against A(H1N1)pdm09 (all ages and settings), VE point estimates ranged from 28% to 46%, higher among children (< 18 years) at 49-77%. Against A(H3N2), overall VE ranged from 2% to 44%, also higher among children (62-70%). Against influenza B/Victoria, overall and age-specific VE were & GE; 50% (87-95% among children < 18 years). Conclusions: Interim results from six European studies during the 2022/23 influenza season indicate a & GE; 27% and & GE; 50% reduction in disease occurrence among all -age influenza vaccine recipients for influenza A and B, respectively, with higher reductions among children. Genetic virus characterisation results and end-of -season VE estimates will contribute to greater under-standing of differences in influenza (sub)type-specific results across studies.