Changes in Clinical Features and Severity of COVID-19 with the Emergence of Omicron Variants: A Shift Towards a Common Disease.

Background: The emergence of the Omicron variant of severe acute respiratory syndrome coronavirus-2 has significantly altered the clinical features and severity of coronavirus disease 2019 (COVID-19). Objective: This study aims to evaluate whether the clinical factors that previously predicted COVID-19 remain valid following the emergence of the Omicron variant. Methods: This cross-sectional study was conducted at Showa University Fujigaoka Hospital from April 2022 to March 2023. A total of 576 patients with suspected COVID-19 were included, of which 258 (44.8%) were diagnosed with COVID-19 based on real-time reverse-transcription polymerase chain reaction tests. Clinical data were collected retrospectively, and multivariate logistic regression was used to analyze factors associated with a COVID-19 diagnosis. Results: Of the 258 patients diagnosed with COVID-19, 60% had mild disease, and the overall severity was lower than in previous reports prior to the emergence of the Omicron variant. In the multivariate analysis, only C-reactive protein (CRP) levels were significantly associated with COVID-19 (odds ratio, 0.3164; 95% confidence interval, 0.2077-0.4819), while factors such as age, sex, body mass index, lactate dehydrogenase, and comorbidities were not significantly associated. Non-COVID-19 cases were primarily bacterial infections, accounting for 57.2% of the non-COVID-19 diagnoses. Mortality rates did not differ significantly between the COVID-19 and non-COVID-19 groups. Conclusion: The clinical characteristics of COVID-19 have become less distinct since the emergence of the Omicron variant, with CRP being the primary marker associated with a COVID-19 diagnosis. As COVID-19 continues to transition towards a more common infectious disease, distinguishing it will become increasingly challenging.