Distinct Clinical and Laboratory Features of COVID-19 in Children During the Pre-Delta, Delta and Omicron Wave

Background: The SARS-CoV-2 variants of concern exhibit distinct features in terms of transmissibility and virulence. This study compared the clinical characteristics of COVID-19 in children during pre-Delta, Delta and Omicron waves. Methods: Medical records of 1163 children <19 years of age with COVID-19 admitted to a designated hospital in Seoul, South Korea, were analyzed. Clinical and laboratory findings during the pre-Delta (March 1, 2020, to June 30, 2021; 330 children), Delta (July 1, 2021, to December 31, 2021; 527 children) and Omicron (January 1, 2022, to May 10, 2022; 306 children) waves were compared. Results: Children during the Delta wave were older and had a higher proportion of fever ≥5 days and pneumonia than children during the pre-Delta and Omicron waves. The Omicron wave was characterized by younger age and a higher proportion of fever ≥39.0 °C, febrile seizure and croup. More children <2 years of age and adolescents aged 10 to <19 years experienced neutropenia and lymphopenia, respectively, during the Delta wave. Children aged 2 to <10 years had a higher incidence of leukopenia and lymphopenia during the Omicron wave. Conclusions: Distinct features of COVID-19 were observed in children during the Delta and Omicron surges. Continuous scrutiny of the manifestations of variants of concern is needed for appropriate public health response and management.