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Objectives: In order to support policymakers in allocating resources, we aimed to assess vaccine effectiveness (VE) of inactivated influenza vaccines (IIVs) available for Italian adults in the 2023/2024 season., Methods: A hospital-based test-negative case-control study was conducted in Genoa between mid-October 2023 and mid-April 2024. Adult (≥18 years) inpatients with prescription of a polymerase chain reaction test for influenza were eligible., Results: Of 1,664 adults analyzed, most (82%) of which were ≥65 years, 114 (6.9%) tested positive for influenza A. Most (92%) cases were caused by subclades 6B.1A.5a.2a and 6B.1A.5a.2a.1 of the A(H1N1)pdm09 subtype. In older adults aged ≥65 years vaccination was effective at 51% (95% CI: 8%, 74%) against any influenza A and 49% (95% CI: 2%, 73%) against A(H1N1)pdm09. Compared with non-vaccinated older adults, VE point estimates for the adjuvanted and, especially, high-dose IIVs were higher than those for the standard-dose non-adjuvanted IIV., Conclusion: The 2023/2024 seasonal influenza vaccination proved moderately effective in preventing hospitalization for laboratory-confirmed influenza. Being more appropriate for older adults, local policymakers and vaccinating physicians should maximize adoption of the enhanced IIVs., Competing Interests: Declaration of competing interest A. D. provided consultancies and received speaker fees from CSL Seqirus, GSK and SD Biosensor and was previously (from 2016 to 2021) an employee of Seqirus S.r.L. A. O. provided consultancies and/or received speaker fees from CSL Seqirus, Moderna, Novavax and SD Biosensor. D. P. provided consultancies for Pfizer and CSL Seqirus and received grants for conducting observational studies from Sanofi, Pfizer, GSK and Viatris. G. I. provided consultancies and/or received grants for conducting experimental and/or observational studies for GSK, Sanofi, MSD, CSL Seqirus and Pfizer. Other authors declare no conflicts of interest., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The effectiveness of seasonal influenza vaccination (SIV) varies from year to year. Interim estimates of vaccine effectiveness (VE) in outpatient settings have suggested that the 2022/23 northern hemisphere SIV was 54 % effective. The main goal of this study was to measure the 2022/23 SIV VE among Italian adults in a hospital setting. The study adopted a retrospective test-negative case-control design and was conducted in a large tertiary hospital (Genoa, Italy) between October 2022 and April 2023. Adult (≥18 years) patients accessing the hospital's Emergency Department for symptoms ascribable to an acute respiratory infection, for which a reverse-transcription real-time polymerase chain reaction test for the detection of influenza virus was prescribed, were potentially eligible. Of 33,692 referrals assessed, 487 patients were included in the study. A total of 13 % of patients were positive for influenza, most of which (63 %) belonged to the A(H3N2) subtype. SIV VE was 57 % (95 % CI: 11-81 %), 53 % (95 % CI: 2-80 %) and 38 % (95 % CI: -34-74 %) against any influenza, influenza A and A(H3N2), respectively. Although no cases caused by A(H1N1)pdm09 and B strains were observed among vaccinated individuals, VE estimates against the latter were imprecise, owing to their low detection rates. In conclusion, the 2022/23 SIV was moderately effective against hospital encounters for laboratory-confirmed influenza., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)