Influenza vaccine effectiveness among Italian adult inpatients, 2024/2025 season

Seasonal influenza vaccination is a recognized means for promoting healthy ageing. In regions like Liguria (Italy), which has the oldest population in Europe, influenza vaccination becomes even more critical. This study aimed to assess influenza vaccine effectiveness (IVE) for the prevention of hospitalization for laboratory-confirmed influenza among Ligurian adults. We conducted a retrospective test-negative case-control study in an inpatient setting, using routinely collected data. Adults (≥18 years) presenting to emergency department from November 4, 2024 through April 27, 2025, who were prescribed a molecular test for the detection of influenza virus and were subsequently hospitalized, were eligible. IVE was estimated via conditional logistic regression. In all, 173 cases and 1166 influenza-negative inpatient controls were included. Most cases were due to influenza A(H1N1)pdm09 (34 %) and A(H3N2) (59 %). In the overall population ̥≥18 years, IVE against hospitalization for any influenza virus was 47 % [95 % confidence interval (CI): 22 %, 63 %]. IVE in older adults ≥65 years was lower (43 %; 95 % CI: 14 %, 62 %) than in younger adults (76 %; 95 % CI: 5 %, 94 %). In an analysis on the effect of prior vaccination, repeat vaccinees were immunized earlier than non-repeat vaccinees. Compared to adults who were not vaccinated in either the previous or current season, those vaccinated in both seasons and current season only, were protected at 48 % (95 % CI: 19 %, 66 %) and 45 % (95 % CI, −17 %; 75 %), respectively. Influenza vaccines available for Italian adults during the 2024/2025 season provided a moderate protection against influenza-related hospitalization. • A retrospective test-negative study was performed among Italian adults. • Vaccine effectiveness against influenza-related hospitalizations was assessed. • The 2024/2025 season in Italy was largely dominated by influenza A viruses. • A moderate vaccine effectiveness of 47 % was established. • In older adults, vaccine effectiveness was lower than in younger adults.