Severity of Respiratory Viral Diseases and the Impacts of Underlying Medical Conditions During the Omicron Subvariant Dominant Epidemics—A Comparative Study of SARS-CoV-2, Influenza Virus and Respiratory Syncytial Virus

After the transition of coronavirus disease 2019 (COVID-19) from a pandemic to an endemic phase, data on respiratory viral infections remain limited. This study compared the clinical outcomes of SARS-CoV-2, influenza virus (INFV), and respiratory syncytial virus (RSV) infections and investigated how underlying medical conditions influence disease severity. During Omicron subvariant dominant periods, we conducted a multicenter, retrospective cohort study including laboratory-confirmed cases of SARS-CoV-2, INFV, and RSV infections in hospitalized patients aged ≥ 19 years. We compared demographic characteristics and clinical outcomes and analyzed the association between underlying comorbidities and severity of infection. A total of 1850 cases with SARS-CoV-2, 98 with INFV, and 63 with RSV infections were analyzed. Notable differences in the occurrence of fever, cough, sputum, and dyspnea were observed among patients with the three different viral infections. Pneumonia was diagnosed more frequently in patients with RSV infection (65.6%) compared to those with INFV infection (42.9%) and SARS-CoV-2 (34.4%) (<i>p</i> < 0.01). For patients with SARS-CoV-2 infection, the risk of pneumonia increased by 47% in the moderate-risk group and 37% in the high-risk group. Among hospitalized patients, pneumonia was more frequently identified in patients with RSV infection, with statistical significance. Furthermore, the presence of medical conditions significantly increased the risk of developing pneumonia.