Background Coronavirus disease 2019 (COVID-19) is generally asymptomatic or mild in otherwise healthy children, however, severe cases may occur. In this study, we report the clinical characteristics of children classified as critical COVID-19 in Korea to provide further insights into risk factors and management in children. Methods This study was a retrospective case series of children < 18 years of age classified as critical COVID-19. Cases were identified by the Korea Disease Control and Prevention Agency surveillance system and medical records were reviewed. Critical COVID-19 was defined as cases with severe illness requiring noninvasive (high flow nasal cannula, continuous positive airway pressure, or bilevel positive airway pressure) or invasive mechanical ventilation, extracorporeal membrane oxygenation (ECMO) or continuous renal replacement therapy (CRRT), between January 20, 2020 and October 7, 2021. Results Among 39,146 cases diagnosed with COVID-19 in subjects < 18 years of age, eight cases (0.02%) were identified as critical COVID-19. The median age was 13 years (range 10 month–17 years) and male-to-female ratio was 1:1. Three children had underlying diseases; one child has asthma and major depressive disorder, one child had Lennox-Gastaut syndrome and one child had mental retardation and was newly diagnosed with type 2 diabetes mellitus with the diagnosis of COVID-19. Among the eight children, seven were obese (body mass index range [BMI] median 29.3, range 25.9–38.2, weight-for-length > 97% for infant) and one was overweight (BMI 21.3). All patients had fever, six patients had dyspnea or cough and other accompanied symptoms included sore throat, headache, lethargy and myalgia. Radiologic findings showed pneumonia within 1–8 days after symptom onset. Pneumonia progressed in these children for 2–6 days and was improved within 5–32 days after diagnosis. Among the eight critical cases, remdesivir was administered in six cases. Steroids were provided for all cases. Inotropics were administered in one case. Six cases were treated with noninvasive mechanical ventilator and three required mechanical ventilator. One case required ECMO due to acute respiratory distress syndrome. All cases were admitted to the intensive care unit and admission period ranged from 9–39 days. Among all critical COVID-19 cases < 18 years of age, there were no fatal cases. Conclusion To develop appropriate policies for children in the COVID-19 pandemic, it is important to monitor and assess the clinical burden in this population.

National cohort data collected during the coronavirus disease 2019 (COVID-19) delta and omicron periods in Korea revealed a lower risk of severe infection in recipients of three doses of the COVID-19 vaccine (adjusted odds ratio [aOR], 0.05–0.08). The risk of death was reduced during the omicron period compared to the delta period (aOR, 0.75; 95% confidence interval, 0.67–0.84).

Background The epidemiology of coronavirus disease 2019 (COVID-19) has changed continuously throughout the pandemic. Methods We analyzed changes in the incidence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection according to the age group in South Korea from February 2020 to December 2021. Results Since the COVID-19 pandemic, the incidence among adults aged ≥ 18 years was higher than all the other age groups in 2020; however, a shift toward younger ages occurred in June 2021. In addition, we found significant changes in epidemiology after the introduction of SARS-CoV-2 vaccines in adults aged ≥ 18 and children 12–17 years. Until recently, children were not regarded as the drive for the pandemic; however, children aged 5–11 and 0–4 years had the highest incidence among all the age groups. Conclusion Therefore, policies for clinical support for an increase in COVID-19 cases among young children and age-specific preventive measures are needed.

Objectives: Household contacts of confirmed cases of coronavirus disease 2019 (COVID-19) are exposed to a high risk of viral transmission, and secondary incidence is an important indicator of community transmission. This study analyzed the secondary attack rate and mRNA vaccine effectiveness against transmission (VET) for index cases (patients treated at home) confirmed to be infected with the Delta and Omicron variants.Methods: The subjects of the study were 4,450 index cases and 10,382 household contacts. Logistic regression analysis was performed to compare the secondary attack rate by vaccination status, and adjusted relative risk and 95% confidence intervals were identified.Results: The secondary attack rate of the Delta variant was 27.3%, while the secondary attack rate of the Omicron variant was 29.8%. For the Delta variant, groups with less than 90 days and more than 90 days after 2 doses of mRNA vaccination both showed a VET of 37%. For the Omicron variant, a 64% VET was found among those with less than 90 days after 2 doses of mRNA vaccination.Conclusion: This study provides useful data on the secondary attack rate and VET of mRNA vaccines for household contacts of COVID-19 cases in South Korea.

Objectives: It is crucial to establish the characteristics of coronavirus disease 2019 (COVID-19) outbreaks at army training centers to develop preventive measures. Therefore, this study aimed to determine the COVID-19 transmission patterns and risk factors in a sequence of outbreaks at an army training center from June to August 2021.Methods: This study included 1,324 trainees at an army training center where outbreaks occurred from June to August 2021. The outbreak was qualitatively analyzed according to the period, attack rate, demographic characteristics, vaccination history, and living areas. An aerodynamic experiment was performed to evaluate aerosol transmission in living areas.Results: Three outbreaks occurred at the army training center from June to August 2021. The first, second, and third outbreaks lasted for 32, 17, and 24 days, and the attack rates were 12.8%, 18.1%, and 8.9%, respectively. Confirmed cases were distributed in all age groups. Recruits and the unvaccinated were at higher risk for COVID-19. The aerodynamic experiment verified the possibility of aerosol transmission within the same living area.Conclusion: COVID-19 transmission at army training centers should be minimized through quarantine and post-admission testing during the latency period as part of integrated measures that include facility ventilation, vaccination, indoor mask-wearing, and social distancing.