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

Objectives This study was conducted to estimate the time-variant reproductive number (Rt) of coronavirus disease 19 in Seoul, South Korea based on either the number of daily confirmed cases or their onset date to monitor the effectiveness of quarantine policies. Methods Using the number of daily confirmed cases from January 23 to March 22, 2020 and their symptom onset date from the official website of the Seoul metropolitan government and the district office, we calculated Rt using the R package EpiEstim. Separate analyses were conducted with the symptom onset date of asymptomatic cases considered as -2, -1, 0, +1, and +2 days of the confirmed date. Results A propagated epidemic curve was found on the basis of the 313 confirmed cases. The daily Rt based on the number of daily confirmed cases peaked at 2.6 on February 20, and decreased to <1.0 starting on March 3. Rt calculated based on the number of daily onset cases showed a similar pattern, although the variation in Rt was greater when using the number of daily confirmed cases. Changing the assumed onset date for asymptomatic cases (from -2 to +2 days of the confirmed date) did not yield meaningful changes in the results. Conclusion Rt can be estimated based on the number of daily confirmed cases, which is available from the daily report of the Korea Centers for Disease Control. Estimating Rt would be useful for continuously monitoring the effectiveness of the quarantine policy at the city and province levels.