Study population and setting
This study included 2,023 confirmed COVID-19 cases in South Korea from January 20 to February 23, 2020 (first time period) and February 24 to April 21, 2020 (second time period). Time periods coincided with the South Korean government’s declaration of public health emergency on February 23, 2020, resulting in subsequent implementation of non-pharmaceutical interventions (enhanced screening and testing in community and clinical settings, quarantining close case contacts, physical distancing measures [implemented beginning March 21, 2020]) designed to mitigate the spread of COVID-19. These strict social distancing measures included cancelling public events, as well as recommendations to avoid social gatherings and leave the household only for essential purposes (i.e., grocery shopping, seeking healthcare). Investigators estimated the effective reproductive number (Rt), a time-varying measure of transmission intensity, using daily case counts to determine the potential impact of combination non-pharmaceutical interventions on the spread of COVID-19.
Summary of Main Findings
In the first time period, the Rt reached a maximum of 2.53 (95% CI: 1.90 – 3.25) secondary transmission events per COVID-19 case. Immediate declines in the Rt were observed 1 week after the emergency declaration on February 23, 2020, reducing to 1.37 (95% CI: 1.27 – 1.47). By February 29, 2020, the Rt fell below 1, indicating falling incidence and declining epidemic spread, and remained <1 for the duration of the observation period. Following the implementation of additional physical distancing measures on March 23, 2020, the Rt declined additionally by 9.75% (95% CI: 7.23 – 12.29%).
The authors modeled the effective reproductive number before and after implementation of various non-pharmaceutical interventions to strengthen attribution of temporal declines in COVID-19 transmission to these interventions.
Given the small number of confirmed cases before February 16, 2020, the authors could not calculate stable Rt estimates prior to this date. Additionally, the authors excluded cases from Daegu-Gyeongsangbuk Province (accounting for approximately 2% of confirmed cases nationally), where superspreading events were documented. The analysis does not support identification of marginal impacts of individual non-pharmaceutical interventions on COVID-19 spread. Though the authors emphasize non-pharmaceutical interventions were implemented without a formal national lockdown, the distancing measures implemented in South Korea were akin to the strictest social distancing measures implemented in other settings, like the United States. Lastly, the mean time of symptom onset between a suspected index case and secondary case was less than zero days in a number of transmission pairs used to estimate the serial interval, which may have impacted downstream model results.
The study offers insights on sequential reductions in COVID-19 transmission corresponding to: 1) declaration of a public health emergency in South Korea; and 2) implementation of physical distancing measures in the absence of a more strict lockdown.
This review was posted on: 25 August 2020