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 studies, 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)typespecific 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 influenza A. Among all ages and settings, VE against influenza 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 ≥ 50% (87-95% among children < 18 years). Conclusions: Interim results from six European studies during the 2022/23 influenza season indicate a ≥ 27% and ≥ 50% reduction in disease occurrence among allage influenza vaccine recipients for influenza A and B, respectively, with higher reductions among children. Genetic virus characterisation results and end-ofseason VE estimates will contribute to greater understanding of differences in influenza (sub)type-specific results across studies.