Association between influenza vaccination and severe outcomes in adults aged ≥ 65 years hospitalized with laboratory-confirmed influenza: a retrospective cohort study, I-MOVE hospital network, Europe, 2015/16–2024/25

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INTRODUCTION: Older adults face an increased risk of influenza-related intensive care unit (ICU) admission and in-hospital death (IHD). Although vaccination reduces hospitalization, its effect on these outcomes remains uncertain. We estimated the association between influenza vaccination and both outcomes among adults aged ≥65 years hospitalized with laboratory-confirmed influenza. RESEARCH DESIGN AND METHODS: We conducted a retrospective cohort study using European I-MOVE hospital network data (2015/16-2024/25); vaccination was defined as vaccine receipt ≥14 days before symptom onset. We estimated pooled-season adjusted risk ratios (aRR) using Poisson regression adjusting for potential confounders. We calculated relative risk reduction (RRR) as (1-aRR)×100, overall and by age group (65-79, ≥80 years). RESULTS: After exclusions, 4,257 patients (five study sites, six seasons) were included. ICU admission occurred in 3% of vaccinated and 5% of unvaccinated patients; IHD occurred in 5% of both groups. RRR was 11% (95% CI: -21-34) against ICU admission (65-79 years: -4%; ≥80 years: 44%). RRR against IHD was 26% (95% CI: 3-43); 17% and 29% in each respective age group. CONCLUSIONS: Vaccination showed little or no association with ICU admission, but was associated with a one-quarter reduction in IHD among hospitalized older adults, reinforcing its benefits against fatal in-hospital outcomes.

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