Transmission onset distribution of COVID-19

•The transmission onset of COVID-19 relative to the symptom onset is noteworthy.•We inferred transmission onset time from 72 infector-infectee pairs with defined contact history.•The median transmission onset was 1.31 days after, and the peak was 0.72 days before the symptom onset.•The pre-symptomatic transmission proportion was 37% (95% CI, 16–52%).•The transmission onset peaked with the symptom onset and the pre-symptomatic transmission proportion is substantial. 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 it is difficult to know who infected whom exactly when. We inferred transmission onset time from 72 infector-infectee pairs in South Korea, either with known or inferred contact dates, utilizing the incubation period. Combining this data with known information of the 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. We estimated the median transmission onset to be 1.31 days (standard deviation, 2.64 days) after symptom onset with a 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). Considering that the transmission onset distribution peaked with the symptom onset and the pre-symptomatic transmission proportion is substantial, the usual preventive measures might be too late to prevent SARS-CoV-2 transmission.