ABSTRACT
Objectives@#This study aimed to examine the scale, characteristics, risk factors, and modes of transmission in a coronavirus disease 2019 (COVID-19) outbreak at a high school in Seoul, Republic of Korea. @*Methods@#An epidemiological survey was conducted of 1,118 confirmed cases and closecontacts from a COVID-19 outbreak at an educational facility starting on May 31, 2021. Indepth interviews, online questionnaires, flow evaluations, and CCTV analyses were used todevise infection prevention measures. Behavioral and spatial risk factors were identified, and statistical significance was tested. @*Results@#Among 3rd-year students, there were 33 confirmed COVID-19 cases (9.6%). Students who used a study room in the annex building showed a statistically significant 4.3-foldelevation in their relative risk for infection compared to those who did not use the study room. Moreover, CCTV facial recognition analysis confirmed that 17.8% of 3rd-year students did not wear masks and had the lowest percentage of mask-wearers by grade. The air epidemiological survey conducted in the study room in the annex, which met the 3 criteria for a closed space, confirmed that there was only 10% natural ventilation due to the poor ventilation system. @*Conclusion@#To prevent and manage the spread of COVID-19 in educational facilities, advance measures that consider the size, operation, and resources of each school are crucial. In addition, various survey methodologies should be used in future studies to quickly analyze a wider range of data that can inform an evidence-based quarantine response.