Time-variant reproductive number of COVID-19 in Seoul, Korea

OBJECTIVES: To estimate time-variant reproductive number (R(t)) of coronavirus disease 19 based on either number of daily confirmed cases or their onset date to monitor effectiveness of quarantine policies. METHODS: Using number of daily confirmed cases from January 23, 2020 to March 22, 2020 and their symptom onset date from the official website of the Seoul Metropolitan Government and the district office, we calculated R(t) using program R’s package “EpiEstim”. For asymptomatic cases, their symptom onset date was considered as -2, -1, 0, +1, and +2 days of confirmed date. RESULTS: Based on the information of 313 confirmed cases, the epidemic curve was shaped like ‘propagated epidemic curve’. The daily R(t) based on R(t_c) peaked to 2.6 on February 20, 2020, then showed decreased trend and became <1.0 from March 3, 2020. Comparing both R(t) from R(t_c) and from the number of daily onset cases, we found that the pattern of changes was similar, although the variation of R(t) was greater when using R(t_c). When we changed assumed onset date for asymptotic cases (-2 days to +2 days of the confirmed date), the results were comparable. CONCLUSIONS: R(t) can be estimated based on R(t_c) which is available from daily report of the Korea Centers for Disease Control and Prevention. Estimation of R(t) would be useful to continuously monitor the effectiveness of the quarantine policy at the city and province levels.