Short-term and long-term risk of death and readmission among adults aged 50+ years admitted with severe acute respiratory infections due to COVID-19, influenza or RSV in Denmark, 2022-2025

INTRODUCTION: Respiratory infections due to COVID-19, influenza and respiratory syncytial virus (RSV) are common causes of morbidity and mortality among adults. We aimed to examine the incidence, severity and mortality of the three viruses among Danish hospitalised adults during three postpandemic years to evaluate the relative impact at population level.

METHODS: We included individuals aged 50 years and older admitted with a severe acute respiratory infection (SARI) due to COVID-19, influenza A or RSV between 23 May 2022 and 18 May 2025. Patients were identified from the national surveillance system for SARI through a combination of relevant International Statistical Classification of Diseases and Related Health Problems-10th edition diagnosis codes combined with a positive molecular test for either SARS-CoV-2, influenza A or RSV. Weekly incidence rates were calculated as number of cases per 100 000 population. For severity indicators, we analysed 30-day in-hospital mortality; 90-day postdischarge mortality; provision of intensive care treatment within 30 days of admission and 90-day risk of readmission.

RESULTS: Weekly incidence rates were higher for COVID-19 during the winter seasons of 2022/2023 and 2023/2024 than for influenza A and RSV, while influenza A dominated during the winter season of 2024/2025. In-hospital mortality across the three study seasons was similar for all three pathogens, while postdischarge mortality at 90 days after discharge was markedly higher for patients with COVID-19, with risk ratios and 95% CIs of 1.49 (1.38 to 1.61) compared with patients with influenza A and 1.36 (1.20 to 1.55) compared with patients with RSV. The risk ratios of readmission at 90 days were similarly higher for patients with COVID-19.

CONCLUSIONS: Although the population-level impact of COVID-19 declined during three postpandemic years, adult patients admitted with COVID-19 consistently faced a higher risk of readmission and a higher mortality during the first 90 days after discharge compared with patients admitted with influenza A or RSV.