Background: Coronavirus disease 2019 (COVID-19) can lead to various complications, including multisystem inflammatory syndrome in children (MIS-C) and post-COVID-19 conditions (long COVID). This study aimed to determine the rates, risk factors and outcomes of MIS-C and long COVID in children previously diagnosed with COVID-19.

Methods: This study was a combined retrospective and prospective cohort study. Patients 0-18 years of age diagnosed with COVID-19 or another respiratory virus infection were enrolled between October 2021 and April 2022. Demographic and clinical data were reviewed. Information on persisting symptoms and their impacts were recorded at 1-3, 3-6 and 6-12 months after infection. Laboratory investigations and chest imaging examinations were performed during follow-up. MIS-C and long COVID were defined according to the Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO) definitions.

Results: A total of 618 patients were enrolled, comprising 437 (70.7%) with COVID-19 and 181 (29.3%) with another respiratory virus infection. At 1-3 months, the rate of persisting symptoms was 16.5% in patients with COVID-19, compared with 1.1% in patients with another respiratory virus infection. The rate of MIS-C was 0.7%. The rate of long COVID according to the CDC and WHO definitions were 20.4% and 13.0%, respectively. Respiratory systems were most affected in long COVID. Age 5-18 years, anosmia during COVID-19, history of pneumonia and infection during the delta and omicron waves were associated with long COVID in children.

Conclusions: Long COVID after COVID-19 in children is uncommon. Children with anosmia and a history of pneumonia during COVID-19 require follow-up for long COVID.