Serial Interval and Transmission Dynamics during SARS-CoV-2 Delta Variant Predominance, South Korea

Abstract

We estimated mean serial interval and superspreading potential for the Delta variant of severe acute respiratory syndrome coronavirus 2 in South Korea. Intervals were similar for the first (3.7 days) and second (3.5 days) study periods. Risk for superspreading events was also similar; 23% and 25% of cases, respectively, seeded 80% of transmissions.

Keywords: COVID-19; Delta variant; SARS-CoV-2; South Korea; coronavirus disease; respiratory infections; severe acute respiratory syndrome coronavirus 2; superspreading; transmissibility; viruses; zoonoses.

Figure 1

Estimated serial interval distribution, incidence of coronavirus disease, and transmissibility during predominance of the Delta variant of severe acute respiratory syndrome coronavirus 2 in South Korea. A) Estimated serial interval distribution for 3,728 infector-infectee pairs. Solid blue line indicates fitted normal distribution; vertical bars indicate the distribution of empirical serial intervals. B) Reported number of confirmed coronavirus disease cases by date of symptom onset. Red vertical dashed line indicates the date of implementation of an enhanced social distancing, including limiting gathering sizes to 4 persons nationwide on July 19, 2021. C) Estimated daily R_t of severe acute respiratory syndrome coronavirus 2 (blue line) with 95% credible intervals (gray shade). Gray horizontal dashed line indicates the critical threshold of R_t = 1. Red vertical dashed line indicates the date of implementation of an enhanced social distancing. R_t, effective reproductive number.

Figure 2

Risk for superspreading events for coronavirus disease during the Delta variant of severe acute respiratory syndrome coronavirus 2 predominance in South Korea. Joint estimates of k and R₀ of coronavirus disease were calculated by using 5,778 pairs (2,169 for period 1 and 3,609 for period 2). The red and blue ovals indicate the bivariate 95% credible region of the estimated k and R₀ for period 1 and period 2. The posterior marginal distributions were plotted in red and blue shaded regions. Period 1, July 11, 2021–July 24, 2021; period 2, July 25, 2021–August 15, 2021. k, overdispersion parameter; R₀, basic reproduction number.