Characteristics and outcomes of adult patients hospitalised with RSV infection, influenza or COVID-19 in Belgium, winter seasons 2023/24 and 2024/25.

BACKGROUNDSevere acute respiratory infections (SARI) cause substantial disease burden in older adults and those with comorbidities.AIMTo inform Belgian vaccination policies, we aimed to compare risk profiles and outcomes of influenza, COVID-19 and RSV infections and examine age-related differences.METHODSAdults (≥ 18 years) hospitalised with SARI in nine Belgian sentinel hospitals during the 2023/24 and 2024/25 winter seasons were included. Patients with laboratory-confirmed RSV infection, influenza or COVID-19 were compared in terms of demographics, comorbidities, complications and outcomes. Adults 18-64 years were compared with those ≥ 65 years. Multivariate analyses were applied.RESULTSAmong 2,806 patients, 535 (19.1%) had RSV infection, 1,694 (60.4%) influenza and 577 (20.6%) COVID-19. Patients with influenza were significantly younger (mean age 68.9 years) and had significantly fewer comorbidities (89.7%) than patients with RSV (73.8 years; 95.8%) or COVID-19 (74.0 years; 93.6%). Patients with COVID-19 experienced significantly fewer complications (40.8%) than patients with RSV (48.9%) or influenza (45.2%). Patients with RSV had higher in-hospital mortality (11.9%) than patients with influenza (7.3%; p = 0.089) or COVID-19 (6.9%; p = 0.037). Across pathogens, younger adults had relatively more comorbidities after adjusting for differences in the general population. They had shorter hospital stays and lower mortality, but higher intensive care unit admission rates.CONCLUSIONPatients with RSV infection had a disease course comparable to patients with influenza or COVID-19 and a higher mortality rate, especially with increasing age. These findings highlight the importance of RSV vaccination in older adults and those with comorbidities, alongside established vaccination strategies for seasonal influenza and COVID-19.