Response to COVID-19 in South Korea and implications for lifting stringent interventions

Abstract

Background: After experiencing a sharp growth in COVID-19 cases early in the pandemic, South Korea rapidly controlled transmission while implementing less stringent national social distancing measures than countries in Europe and the USA. This has led to substantial interest in their "test, trace, isolate" strategy. However, it is important to understand the epidemiological peculiarities of South Korea's outbreak and characterise their response before attempting to emulate these measures elsewhere.

Methods: We systematically extracted numbers of suspected cases tested, PCR-confirmed cases, deaths, isolated confirmed cases, and numbers of confirmed cases with an identified epidemiological link from publicly available data. We estimated the time-varying reproduction number, R_t, using an established Bayesian framework, and reviewed the package of interventions implemented by South Korea using our extracted data, plus published literature and government sources.

Results: We estimated that after the initial rapid growth in cases, R_t dropped below one in early April before increasing to a maximum of 1.94 (95%CrI, 1.64-2.27) in May following outbreaks in Seoul Metropolitan Region. By mid-June, R_t was back below one where it remained until the end of our study (July 13th). Despite less stringent "lockdown" measures, strong social distancing measures were implemented in high-incidence areas and studies measured a considerable national decrease in movement in late February. Testing the capacity was swiftly increased, and protocols were in place to isolate suspected and confirmed cases quickly; however, we could not estimate the delay to isolation using our data. Accounting for just 10% of cases, individual case-based contact tracing picked up a relatively minor proportion of total cases, with cluster investigations accounting for 66%.

Conclusions: Whilst early adoption of testing and contact tracing is likely to be important for South Korea's successful outbreak control, other factors including regional implementation of strong social distancing measures likely also contributed. The high volume of testing and the low number of deaths suggest that South Korea experienced a small epidemic relative to other countries. Caution is needed in attempting to replicate the South Korean response in populations with larger more geographically widespread epidemics where finding, testing, and isolating cases that are linked to clusters may be more difficult.

Keywords: COVID-19; Contact tracing; Public health interventions; Reproduction number; South Korea.

Fig. 1

Incidence of confirmed cases by date of the report from daily KCDC press releases with key intervention timings (a) and deaths (b). Interventions are separated into border control, testing, isolation and social distancing. Sources of interventions: KCDC press releases, Ministry of Health and Welfare, and Ministry of Education

Fig. 2

Estimates of time-varying effective reproductive number R_t based on confirmed cases by date of the report and accounting for imported cases. R_t is estimated using a sliding window of 7 days and an uninformative prior distribution with mean 1 and standard deviation 10. The black line represents the posterior mean value of R_t, and the shaded region shows the 95% credible interval (CrI). The dotted horizontal line shows R_t = 1

Fig. 3

New and cumulative numbers of people being tested in South Korea from the beginning of the COVID-19 outbreak on January 20 until July 13, 2020. Grey bars represent the daily number of tests by date of the report, and black line shows the cumulative number of tests conducted over time. Key changes in testing are also presented. Source: KCDC press releases

Fig. 4

Rolling 7-day average of the confirmed cases per number of people tested until July 13, 2020. The dashed vertical line indicates the date on which South Korea changed its testing protocol from testing suspected cases with contact with a case or travel history to a country with infections to testing all suspected cases (February 20). Source: KCDC press releases

Fig. 5

a Cumulative proportion of confirmed cases by epidemiological link and/or origin from March 25 to July 13. The proportion of cumulative confirmed cases that are linked to existing cases or imported (white line) as opposed to “under investigation” (which includes apparent sporadic cases). Linked cases are broken down into whether they are connected to an imported case, part of the Shincheonji cluster, a smaller cluster or a non-cluster contact of a confirmed case. b Non-cumulative proportion of new cases reported during the 2 weeks prior to the date on the x-axis, by type of case. Source: KCDC press releases

Fig. 6

Isolation dynamics in South Korea. The number of confirmed cases currently in isolation per day. Source: KCDC press releases