[Sixth wave of COVID-19 pandemic: Epidemiological survey in children]

Objectives　At the beginning of the COVID-19 pandemic, the number of infected children was lower than that of adults. Most cases were transmitted in the family, asymptomatic, and severe cases were rare. In the sixth wave in Japan the number of infected children increased sharply after the Omicron variant had been replaced in December 2021, which had a substantial influence in maintaining social and medical functions. Furthermore, few reports on child death in the country have raised concerns among parents. However, no literature has elucidated the epidemiological characteristics of the Omicron variant in children. In this study, we aimed to clarify them during the sixth wave in Japan.Methods　We analyzed the data of 28,086 COVID-19-infected patients those were registered in the Yamashirokita Public Health Center between January 15, 2022 and May 31, 2022. The cumulative incidence and hospitalization rate were compared between the age groups <15 and those >15 years based on the databases compiled by our public health center and the Kyoto prefecture government. In addition, we analyzed the background, length of hospitalization, and clinical symptoms of 24 patients based on active epidemiological investigation, health observations, and discharge reports submitted from medical facilities.Results　Of the 52,897 residents <15 years (pediatric population is 12.3%), 15.1% (7,980 cases) were infected, and children accounted for 28.4% of all-age infected patients. Among them, 24 were hospitalized (0.3% of children with COVID-19, 0.04% of the child population). Conversely, of the 377,093 residents aged ≥15 years, 5.3% (20,106 patients) were infected. Among them, 1,088 were hospitalized (5.4% of COVID-19 patients, 0.28% of the adult population). For 24 hospitalized children, 22 (91.6%) had mild cases and 2 (8.3%) had moderate cases, and no severe case was noted based on the criteria of severity in Japan's COVID-19 medical care guidelines. Two patients (8.3%) were hospitalized for treatment of other diseases. The median of hospital stay was 3.5 days, and 20 patients (83.3%) were discharged home during the recuperation period.Conclusions　The cumulative incidence of children with COVID-19 in the sixth wave was 15.1%, approximately three times higher than that of the older patients; however, no severe case was observed in children.