The distribution of the transmission onset of COVID-19 relative to the symptom onset is a key parameter for infection control. It is often not easy to study the transmission onset time, as is difficult to know who infected whom exactly when. Aim Based on the solid epidemiologic data, we estimate the transmission onset, incubation period, and serial interval distribution of COVID-19 in South Korea. Methods We inferred transmission onset time from 72 infector-infectee pairs in South Korea, either

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... with known or inferred contact dates by means of incubation period. Combining this data with known information of infector's symptom onset, we could generate the transmission onset distribution of COVID-19, using Bayesian methods. Serial interval distribution could be automatically estimated from our data. Results We estimated the median transmission onset to be 1.31 days (standard deviation, 2.64 days) after symptom onset with peak at 0.72 days before symptom onset. The pre-symptomatic transmission proportion was 37% (95% credible interval [CI], 16-52%). The median incubation period was estimated to be 2.87 days (95% CI, 2.33-3.50 days) and the median serial interval to be 3.56 days (95% CI, 2.72-4.44 days). Conclusion Considering the transmission onset distribution peaked with the symptom onset and the pre-symptomatic transmission proportion is substantial, the usual preventive measure might be too late to prevent SARS-CoV-2 transmission.