Transmission dynamics and control of two epidemic waves of SARS-CoV-2 in South Korea

Abstract

Background: After relaxing social distancing measures, South Korea experienced a resurgent second epidemic wave of coronavirus disease 2019 (COVID-19). In this study, we aimed to identify the transmission dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and assess the impact of COVID-19 case finding and contact tracing in each epidemic wave.

Methods: We collected data on COVID-19 cases published by local public health authorities in South Korea and divided the study into two epidemic periods (19 January-19 April 2020 for the first epidemic wave and 20 April-11 August 2020 for the second epidemic wave). To identify changes in the transmissibility of SARS-CoV-2, the daily effective reproductive number (R_t) was estimated using the illness onset of the cases. Furthermore, to identify the characteristics of each epidemic wave, frequencies of cluster types were measured, and age-specific transmission probability matrices and serial intervals were estimated. The proportion of asymptomatic cases and cases with unknown sources of infection were also estimated to assess the changes of infections identified as cases in each wave.

Results: In early May 2020, within 2-weeks of a relaxation in strict social distancing measures, R_t increased rapidly from 0.2 to 1.8 within a week and was around 1 until early July 2020. In both epidemic waves, the most frequent cluster types were religious-related activities and transmissions among the same age were more common. Furthermore, children were rarely infectors or infectees, and the mean serial intervals were similar (~ 3 days) in both waves. The proportion of asymptomatic cases at presentation increased from 22% (in the first wave) to 27% (in the second wave), while the cases with unknown sources of infection were similar in both waves (22 and 24%, respectively).

Conclusions: Our study shows that relaxing social distancing measures was associated with increased SARS-CoV-2 transmission despite rigorous case findings in South Korea. Along with social distancing measures, the enhanced contact tracing including asymptomatic cases could be an efficient approach to control further epidemic waves.

Keywords: COVID-19; Epidemiology; SARS-CoV-2; Social distancing measure; Transmission.

Fig. 1

Incidence of COVID-19 and transmissibility of SARS-CoV-2 in South Korea. A The reported number of confirmed COVID-19 cases by symptom onset date outside of the Daegu-Gyeongsangbuk region in South Korea. The dates of first clinical assessment were used for cases who were asymptomatic at presentation. There were key events against the spread of SARS-CoV-2 including a general election (on 15 April 2020), relaxing strict social distancing measures (from 20 April 2020) and relaxing the social distancing measure further (from 6 May 2020), distribution of COVID-19 relief funds to the general public (from 11 May 2020) and resuming school (from 20 May 2020). B Estimated daily R_t of SARS-CoV-2 in blue line with 95% credible interval in light grey shaded area. The red horizontal dashed line indicates the critical threshold of R_t = 1. The vertical grey dashed line divided the study period on first epidemic waves (19 January–19 April 2020) and second epidemic waves (20 April–11 August 2020). Notes: COVID-19 = coronavirus disease 2019, SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2, R_t = effective reproductive number

Fig. 2

Types of clusters in South Korea in the first and second epidemic waves. Temporal distribution of the clusters of COVID-19 cases outside of the Daegu-Gyeongsangbuk region in South Korea. The vertical grey dashed line divided the study period on first epidemic wave (19 January–19 April 2020) and second epidemic wave (20 April–11 August 2020)

Fig. 3

Serial interval distribution of SARS-CoV-2 in the first and second epidemic waves in South Korea. The estimated serial interval distribution was analysed by using the 708 infector-infectee pairs. The vertical bars indicate the empirical probability density of serial interval calculated by constructing transmission pairs from illness onset of confirmed cases and black lines indicate fitted normal distribution (accounting for the possible negative serial intervals/pre-symptomatic transmissions and symmetric pattern of empirical density). Infector who reported symptoms onset in the first epidemic wave (19 January–19 April 2020; 345 pairs) (A), and second pandemic wave (20 April–11 August 2020; 363 pairs) (B). The left of vertical dashed line in grey indicates definite pre-symptomatic transmission

Fig. 4

Age-specific transmission matrix in South Korea A Infector-infectee matrix for the first epidemic wave (19 January–19 April 2020). The colour in each cell represents the probability of infector-infectee pairs of the respective ages. B Infector-infectee matrix for the second epidemic wave (20 April–11 August 2020)

Fig. 5

The proportions of asymptomatic and unlinked local cases of COVID-19 in South Korea. A Age-specific proportions of COVID-19 cases who were asymptomatic at the time of presentation in South Korea. B Age-specific proportion of infections of an unknown origin in South Korea. The points were average proportions over the epidemic wave and the vertical bars indicate 95% confidence intervals estimated by the binomial method. Notes: COVID-19 = coronavirus disease 2019