Influenza Vaccine Effectiveness Among Italian Adults: A Test-Negative Case–Control Study from an Italian Southern Province During the 2024/25 Winter Season

Background: In Italy, seasonal flu vaccination was recommended for all children aged 6 months to 6 years, all adults aged ≥60 years and specific at-risk groups. Influenza vac-cine effectiveness (IVE) varies from season to season. It can be related to several factors: possible mismatches between circulating virus strains and vaccine composition, the host immune response and the interval between the vaccine administration and the epidemic peak. The present study aimed to estimate IVE against acute respiratory infections (ARIs) caused by influenza during the 2024/25 season in the Province of Pescara, Abruzzo Region, Southern Italy. Methods: A retrospective test-negative case–control study was conducted in the tertiary hospital of the province of Pescara, Abruzzo region, Southern Italy. The study considered all naso/oropharyngeal swabs performed between 15 October 2024 and 1 April 2025. Results: During the study period, 5452 tests were performed and met the inclusion criteria. Among them, 637 tested positive for influenza (11.68%) and 4815 (88.32%) were considered controls. The IVE estimated using the fully adjusted model showed an overall effectiveness of 41.36% (from 1.08 to 57.29%). Considering influenza type, IVE against influenza A was 37.23%, whereas IVE against influenza B was 77.28%. Conclusions: The 2024/2025 season was characterized in our Italian region by a delayed start in flu circulation, with a concomitant and prominent co-circulation of all virus types. Available vaccines provided a moderate protection against laboratory-confirmed influenza, with an estimated VE of 41%.