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Coronavirus Disease-19: Summary of 2,370 Contact Investigations of the First 30 Cases in the Republic of Korea

Our take —

This study reported the secondary attack rates of early COVID-19 cases in South Korea. Findings from the initial 30 cases and 2,370 contacts display how effective testing and contact tracing has been in South Korea at controlling the virus, as the paper reported lower secondary attack rates compared to other papers (0.6% overall and 7.6% within households). Although these estimates may include an excess of contacts, it serves as a potential model for how other municipalities can conduct contact tracing in preventing transmission.

Study design


Study population and setting

The sample includes data the first 30 COVID-19 cases reported in South Korea and 2,370 contacts between January 24, and March 10, 2020.

Summary of Main Findings

Out of the 2,370 reported contacts, 12 became infected, resulting in a secondary attack rate of 0.55% (95% CI: 0.31–0.96). There were 119 household contacts, out of whom 9 contracted the virus, resulting in a household secondary attack rate of 7.56% (95% CI 3.73–14.26). The secondary attack rate among males was higher than that among females (0.75, 95% CI 0.38–1.47 compared to 0.38, 95% CI 0.16–0.89, respectively).

Study Strengths

South Korea had one of the earliest and most exhaustive contact tracing programs in the pandemic, thus results provide insights into pandemic control.


Initially contacts were defined as someone who was within 2 meters of a case, however the definition was later replaced with being a contact of a case regardless of exposure level. This dilutes the contact definition so secondary attack rates may be artificially lower as a result. Sociodemographic risk factors aside from age and sex were not collected.

Value added

They study provides information concerning how South Korea conducted contact tracing (i.e. quarantined asymptomatic contacts for 14 days, and called contacts everyday twice a day for the duration of the quarantine). This, and the relatively low secondary attack rates, demonstrate the benefits of aggressive contact tracing efforts.