Background
Containment, suppression, and mitigation were proposed as coronavirus disease 2019 (COVID-19) pandemic response strategies [
1]. Containment aims to eliminate community transmission (zero incidence for certain period of time beyond the latent period) via stringent interventions such as lockdown, border closure, and extensive tracing and large-scale testing [
1]. Because containment is difficult to sustain in the long term, many countries implemented a suppression or mitigation strategy. Mitigation aims to minimize damage to high-risk populations and so allows a time-varying reproduction number (R
t) > 1. By contrast, suppression aims to reduce R
t below 1 to minimize transmission [
1,
2]. R
t is the average number of secondary cases of an infector during his or her infectious period and can be controlled by countermeasures and behavioral changes [
3]. Lockdown, social distancing, mandatory mask wearing, restrictions on flights from high-risk countries, and temporary border closure were implemented in the United States, Argentina, and Uganda [
1]. In addition, contact tracing and testing were proposed as essential case-based interventions in suppression strategy, but the approaches used differed according to national capabilities [
1]. South Korea implemented a suppression strategy, which did not encompass lockdown or border closure.
South Korea’s COVID-19 control was remarkable compared to other countries that adopted a suppression strategy. In 2020, South Korea exhibited significantly lower daily new cases per million compared to the United States and Argentina, with maximum figures of 139, 4,489, and 2,109 cases per million, respectively. Additionally, the average daily new cases per million in South Korea were also notably lower than those observed in the United States and Argentina. South Korea minimized the number of confirmed cases by its suppression strategy through extensive contact tracing and large-scale testing (the 3Ts; tracing, testing, treatment) [
4,
5]. South Korea achieved noteworthy outcomes, despite not implementing a rigorous containment strategy [
4,
5].
Since the effectiveness of contact tracing is determined by the basic reproduction number (R
0) and fraction of asymptomatic infection, contact tracing alone cannot counter COVID-19, which has high overall and silent transmission rates [
6]. As a complementary measure, appropriate testing may be important. However, combined tracing and testing strategies have not been formulated [
1,
2,
7], so a framework to maximize the effect of tracing and testing is needed. South Korea can serve as a reference for such a suppression strategy.
We performed a review of typical examples of South Korea’s response to COVID-19. In addition, we developed a conceptual model to explore effective tracing and testing strategies. The objectives of this study were to suggest general criteria for tracing and testing based on South Korea’s experience, and to propose a framework to assess tracing and testing.
Methods
This study followed the PRISMA-ScR (Preferred Reporting Items for Systematic Review and Meta-Analyses for Scoping Reviews) guidelines [
8]. We reviewed papers on South Korea’s response to COVID-19 to capture the concept of tracing and testing. Papers that addressed the epidemiological investigation process in South Korea from 2020 to 2021 with the number of tests and cases, were included. In addition, a conceptual model was developed based on the concept of tracing and testing. SEIR model was used, and quarantine was included to the model for further understanding the process of tracing and testing. We gathered COVID-19 risk indicators from KDCA (Korea Disease Control and Prevention Agency) press releases issued between June 2020 and February 2022 to validate the accuracy of our developed model’s hypotheses.
Search strategy
The search terms were combined with terms related to South Korea’s response to COVID-19. A search of studies on databases was performed on 17th October 2021, including Pubmed and Embase. The detail search terms for Pubmed are presented in Supplementary material (Table S
1).
Inclusion and exclusion criteria
In this review, we included studies on South Korea’s response to COVID-19 outbreaks in 2020. Studies that did not include examples of South Korea and did not report the number of confirmed cases and tests were excluded. Additionally, studies not written in English were excluded. Conference abstracts, review paper, letters, editorials, or article comments were excluded. The detail inclusion and exclusion criteria are presented in Supplementary material (Table S
2).
Screening
An author firstly screened each study by title and abstract according to inclusion criteria using reference management software, EndNote 20.2.1 version. After the first screening, another author independently conducted the second screening. The full text of the title and abstract screened studies was reviewed by all authors.
To clarify the tracing and testing strategies of South Korea, the number of cases and tests were extracted from the reviewed literatures. The process of tracing and testing in each study were extracted to describe and understand the strategic changes of South Korea’s response in 2020.
Discussion
In this study, we evaluated tracing and testing in South Korea by analyzing the response to COVID-19 outbreaks. In South Korea, forward and backward tracing were implemented. In addition, group tracing combined with preemptive testing and open testing were conducted to overcome the limitations of conventional contact tracing. We proposed the SEQIR model to explore the properties of case types according to tracing and testing strategies. In the model, the confirmed cases were classified into I1–I4, the proportions of which can be used as tracing and testing performance indicators.
In South Korea, simultaneous forward and backward tracing was an effective countermeasure for COVID-19—backward tracing can identify cases missed by forward tracing (Fig.
1c). This is consistent with prior studies that bidirectional tracing allows the detection of hidden transmission paths [
18,
20]. Moreover, bidirectional tracing was superior for controlling the spread of COVID-19 compared with forward tracing alone [
18,
20]. The proportion of I
1 in South Korea remained almost above 90% from March 2020 to April 2020, which was not included in Fig.
4. This shows that South Korea proactively identified almost all cases by bidirectional contact tracing in the early stage of pandemic as shown in Fig.
2.
The case types in this study were consistent with South Korea’s risk assessment indicators. The proportion of I
1 is identical to the timely quarantined proportion (TQP) proposed previously, and can be used to assess the effects of epidemiological investigation, testing, and quarantine [
28]. It is necessary to monitor trends in I
1 to prevent the spread of COVID-19.
Maintaining a high proportion of I
1 is a challenge for many countries. Furthermore, maintaining a high proportion of I
1 during the period of delta variant predominance was problematic because of its high transmission rate and ability to escape the immune system. The emergence of a new variant can increase the proportion of untraced cases (I
3). An unlinked case is a confirmed case with no link to the infector [
29,
30]. A confirmed case discovered by group tracing and preemptive testing can be classified as an unlinked case, but not as an untraced case, and remains controllable. Reducing the untraced proportion is another major challenge, but can be achieved by increasing the proportion of I
2.
Proactive and fast tracing is necessary to increase I
2. However, as mentioned above, the emergence of new variants can hamper the tracing of individuals suspected of having close contact with confirmed cases. In this case, group tracing and preemptive testing is a feasible alternative strategy to identify super-spreaders and reduce cluster size. During the period of delta variant predominance (after 2021 July) in South Korea, I
1 decreased, but I
3 remained < 40% (Fig.
4). According to the proposed model, this was achieved by group tracing and preemptive testing.
I
4 is one of the major concerns to control the COVID-19 transmission. There are several studies on the estimates of I
4. Lee et al. estimated the proportion of undetected case of COVID-19 in South Korea as 5.8% (5,200/89,244) to 64% (139,900/218,744) using data as of 2nd February 2021, and a probabilistic model they developed [
31]. A modeling study conducted by Huo et al. estimated the proportion of asymptomatic and undetected case in Wuhan, China as 22.4% (14,448/64,454) [
32]. A systematic review which analyzed 79 studies, estimated the proportion of asymptomatic case as 20% (95% C.I 17%25%) [
33]. Additionally, these studies revealed that I
4 has transmissibility [
31,
32], and this was shown in the reviewed studies. The sources of infection of index cases in S. Church, call center, spa facility, and Itaewon nightclubs outbreaks were not identified [
10,
11,
13,
14]. Therefore, it is necessary to consider in the response planning not only the identified cases, but also the unidentified cases.
Unlike previous works, this study described the tracing and testing process in detail. In addition, the proposed model may be useful for other countries. However, we did not address the social distancing and vaccination policies that were instrumental for flattening the COVID-19 curve. In addition, statistical analysis of empirical data was not performed because the current study focused on conceptual analysis of South Korea’s COVID-19 tracing and testing strategies. Lastly, as this was not a systematic review, it did not include all articles that analyzed South Korea’s response to COVID-19.
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