Participants
Inclusion criteria is a child aged 7-18 years old, previously diagnosed with COVID-19 infection by an evidence of viral antigen detection between 1-6 months prior to the study, and the child must be able to perform acceptable spirometry in accordance with the American Thoracic Society (ATS) / European Respiratory Society (ERS) criteria8. Exclusion criteria is any of the followings; 1) a history of smoking, 2) respiratory infection within four weeks prior to the assessment, 3) using corticosteroids within four weeks prior to the assessment, and 4) presence of other medical conditions that may affect pulmonary function, including asthma, restrictive lung diseases and neuromuscular weakness. Children who met the criteria were invited to participate in this study via telephone.