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• In a national cohort of over 10,273 patients with COVID-19, more than 60% of all cases in South Korea reported no symptoms at the time of diagnosis. • Our findings suggest that symptom-based screening alone may fail to control transmission during the infected but asymptomatic stage. • Expanding criteria for contact tracing and testing should be urgently considered. To delineate clinical characteristics of asymptomatic and symptomatic patients confirmed with COVID-19 in South Korea. Data were obtained from the Korean National Health Insurance Service database linked to the Korea Centers for Disease Control and Prevention data. Among 10,237 patients (mean [SD] age, 45.0 [19.8] years; 60.1% female) who met the eligibility criteria for the study, 6,350 (62.0%) patients were asymptomatic, and 3,887(38.0%) patients were symptomatic. The mean and median age were similar between asymptomatic and symptomatic patients. Notably, we observed a U-shaped association between age group and the proportion of asymptomatic patients, with the nadir at 57.3% in the 40-49 age group. This U-shaped distribution was largely similar between men and women. The overall prevalence of asymptomatic individuals was higher, regardless of sex, residential area, income levels, and comorbid conditions. In this national cohort of over 10,000 patients with COVID-19, more than 60% of all cases in South Korea reported no symptoms at the time of diagnosis. Expanding criteria for contact tracing and testing to capture potential transmission before symptom onset should be urgently considered to inform control strategies for COVID-19. [ABSTRACT FROM AUTHOR]