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Contact Tracing during Coronavirus Disease Outbreak, South Korea, 2020

Our take —

This is a study from the national South Korean contact tracing program among 59,073 COVID-19 contacts in the country from January to May 13, 2020. They identified 5,706 index patients, 10,592 household contacts, and 48,481 non-household contacts. 11.8% (n=1248) of household contacts tested positive, compared to 1.9% (921) of non-household contacts who went on to develop symptoms and be tested. While 20 to 29 year-olds made up the majority of index cases, the highest infection rate among household contacts was when the index case were school-aged children. The study used different testing protocols for household and non-household contacts, so these groups may not be wholly comparable. However, this is a nationally representative study, and one of the first to describe contact tracing throughout the country.

Study design

Cross-Sectional

Study population and setting

The study describes the national COVID-19 contact tracing program and age-stratified case counts in South Korea to inform public health policy efforts. The study identified 5,706 index patients and 59,073 contacts for surveillance. Through tracing, by May 13, 2020, 10,962 total cases had been reported across the country and tested by the Korea Centers for Disease Control and Prevention. Index cases were defined as a laboratory-confirmed case, or the first documented case in a given cluster under investigation. High-risk groups were defined as household contacts of COVID-19 patients and healthcare personnel, and were routinely tested. Non-high risk groups were tested only if they developed symptoms. Exposed individuals who were otherwise not considered high-risk were instructed to self-quarantine for 14 days, with twice-daily active surveillance by public health workers. A “detected case” was defined as a non-index case that had symptom onset after contact with a confirmed index patient.

Summary of Main Findings

Of 59,073 contacts identified, 10,592 were household contacts (17.9%) compared to 48,481 non-household contacts identified. Of household contacts, 1,248 went on to become cases (11.8%, 95% CI: 11.2 – 12.4), compared to 921 (1.9%, 95% CI: 1.8 – 2.0%) non-household contacts. Index patients of both household and non-household contacts were most likely to be aged 20 to 29 years (n=15,810, 26.7%), followed by 50 to 59 years (n=11,353, 19.2%). For non-household contacts, the detection of COVID-19 was higher for cases with index patients >40 years of age. The highest infection rate was for household contacts with index cases who were school-aged children (18.6%, 95% C: 14.0 – 24.0%), with the lowest infection rate occurring among household contacts of school-aged children during the school closures (5.3%, 95% CI: 1.3 – 13.7%).

Study Strengths

This study used a robust contact tracing system in order to identify potential cases from both household and non-household contacts. All 59,073 contacts were followed for at least 8 days. All cases also were tested by RT-PCR, allowing for more accurate enumeration of cases. Results were disaggregated by household and non-household contacts, and presented as age-stratified estimates, owing to the fact that age categories and exposure route likely impact infection. With such a large sample size from the national program, these results are likely representative of the target population of South Korea.

Limitations

The study was limited in that asymptomatic, non-high-risk contacts were likely undercounted, though intensive follow-up of non-high-risk, non-symptomatic contacts should have increased the likelihood that pre-symptomatic cases would have eventually been detected. Authors was also unable to determine where transmission occurred, and whether household contacts may have, instead, been exposed outside the home. They presented results disaggregated by household versus non-household contacts due to the different testing strategies used between these groups, but this resulted in the infection rate between these groups no longer being fully comparable.

Value added

This study reported results from the national South Korean contact tracing program, and serves as an important reference for other countries seeking to implement similar contact tracing protocols.

This review was posted on: 28 July 2020