Introduction

Since the first reported cases of coronavirus disease 2019 (COVID-19) in the Republic of Korea, there have been 30 confirmed cases as of February 17^th, 2020. The epidemiological and clinical characteristics of 28 cases have been previously reported [1].

Contact tracing is essential for containing COVID-19 within the community. In Korea, an established system exists where public health centers conduct epidemiological investigations and instigate the quarantine/isolation of suspected cases, thus interrupting the line of transmission. This approach has been particularly successful in containing COVID-19 in the early phase of the outbreak.

Here, we summarize the investigation into the first 30 cases of COVID-19, and the 2,370 individuals who came into contact with these original 30 cases in Korea. The study includes calculation of secondary attack rates among different age groups and modes of transmission of COVID-19.

Materials and Methods

Demographic, epidemiological, and early clinical information were retrieved from COVID-19 reporting and surveillance data from Korea Centers for Disease Control and Prevention (KCDC). As in the previous report, the patient’s age was provided on the date of diagnosis, and key indicators were identified by an epidemiological investigator who participated in the field investigation and the epidemiological investigation team [1]. Data presented in this study may change depending on further epidemiological investigations.

The working definition for “close contact (or high risk exposure)” was being within 2 meters of a COVID-19 case and “daily contact (or low risk exposure)” was defined as having proximity with a person who was a confirmed COVID-19 case, without having had close contact. The classification was then repealed and was integrated into “contact (regardless of level of exposure).” All contacts who were asymptomatic were mandated to self-quarantine for 14 days and were put under active surveillance by public health workers who called twice a day to check presence of fever or respiratory symptoms. A smartphone-based, “self-assessment app,” was introduced as an additional tool to track symptoms of COVID-19.

Results

Between January 24^th and March 10^th, a total of 2,370 individuals were contact traced to the first 30 cases of COVID-19. There were 12 individuals who contracted COVID-19, resulting in a secondary attack rate of 0.55% (95% CI 0.31–0.96; Table 1). Males had a higher secondary attack rate (0.75, 95% CI 0.38–1.47) than females (0.38, 95% CI 0.16–0.89). Of 119 household contacts, 9 individuals had coronavirus infections which resulted in a secondary attack rate of 7.56% (95% CI 3.73–14.26; Table 1). The number of traced individuals from a case ranged from 15 to 649 people and the mean monitored time per person ranged from 5.7 to 31.3 days (Table 2).

Discussion

Contact tracing of 2,370 individuals from the first 30 COVID-19 cases in Korea indicated that the risk of symptomatic cases from transmission to contacts was low at 0.55% (95% CI 0.31–0.96). However, the findings also suggested that the transmission of COVID-19 was significant among household contacts, which is in line with other reports. In the earlier reports, familial clusters of COVID-19 had been reported and household transmission was thought to be a major driver in the spread of the outbreak in the community [2,3]. Of the first 262 COVID-19 cases in Beijing, China, 133 (50.8%) were family cluster cases [4]. In the US, active symptom monitoring was performed for 445 close contacts of the 12 cases with travel-related COVID-19, resulting in symptomatic cases with a secondary attack rate of 0.45% (95% CI, 0.12–1.6) among all contacts, and 10.5% (95% CI, 2.9–31.4) among household members [5].

This study suggested that contact tracing was critical in the containment of the COVID-19 outbreak in the early phase in Korea. Contact tracing relies on other concurrent aspects of the COVID-19 containment strategies such as investigating, classifying, tracking, and managing contacts by identifying the patient’s route. A mathematical model predicted that highly effective contact tracing and quarantine was sufficient enough to control a new outbreak of COVID-19 in most scenarios [6]. In Korea, various tracking measures such as the history of clinic visits, GPS of cell phones, credit card transaction logs, and CCTV have been used to complete the contact tracing of COVID-19 cases [7].

There are certain limitations that should be considered in this study. Firstly, this is a summary of the first 30 cases of COVID-19 in Korea, when containment of cases and contacts was the mainstay of the control strategy. Following the increase in the number of cases above 30, the strategy shifted from containment to mitigation, which is now applied in many parts of the world. Secondly, potential risk factors such as the characteristics of a household and other transmission routes were not assessed and would merit further study. However, the observations reported in this study underscores the infectious nature of COVID-19 in households and identify the main drivers that facilitate secondary transmission in the community.

This summary identifies the overall risk of symptomatic transmission of COVID-19 in the early phase of the disease. Implementation of basic infection control practices, such as isolation of family members who are symptomatic, and enhancement of hand hygiene measures, which may lead to a reduction in transmission of COVID-19. Future analyses should attempt to incorporate screening of COVID-19 and serology in symptomatic and asymptomatic contacts.

Acknowledgements

Acknowledgments

We thank the relevant ministries, including the Ministry of Interior and Safety, Si/Do and Si/Gun/Gu, medical staffs in health centers, and medical facilities for their efforts in responding to the COVID-19 outbreak.

Article information

Conflicts of Interest

There was no conflicts of interest to declare.

COVID-19 National Emergency Response Center, Epidemiology and Case Management Team: Korea Centers for Disease Control and Prevention (KCDC):

Eun Kyeong Jeong

Center for Disease Prevention, KCDC: Ok Park, Young Joon Park, Shin Young Park, Young-Man Kim, Jieun Kim, Joongho Jo, Jungwoo Kim, Taeyoung Kim

Center for Public Health Emergency Preparedness and Response, KCDC: Jin Gwack, Jin Lee, Eunmi Park, Seong Sun Kim, Junghee Hyun, Boyeong Ryu, Yoon Suk Jang, Hwami Kim, Yeowon Jin, Hyeyoung Lee, Sangeun Lee, Seung Hwan Shin, Seung Woo Choi, Daehyeon Ko

Center for Infectious Disease Control, KCDC: Dongwook Kim, Byoung-Hak Jeon

Bureau of Health Policy, Ministry of Health and Welfare:

Unna Kim

Department of Social and Preventive Medicine, Hallym University College of Medicine: Young June Choe

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Figure & Data

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