2.1 Study participants
This was a retrospective study and conducted in Xinhua Hospital, a large tertiary hospital in Shanghai, China. The inclusion criteria for the study were: pneumonia patients admitted to the Department of Pediatric Pulmonology of Xinhua Hospital from March to August 2020 (during the COVID-19 pandemic when all students studied online at home and strengthened personal protection) and in the same period of 2019 (prior to the COVID-19 pandemic when all students studied at school) ; negative for COVID-19; and aged 3 months to 16 years. Children were diagnosed with pneumonia or severe pneumonia based on the criteria recommended by the 2011 pediatric community-acquired pneumonia guidelines18. Since the COVID-19 pandemic, all pneumonia patients were screened for COVID-19 using real-time reverse transcriptase polymerase chain reaction and each recruited child was confirmed negative for COVID-19. Patients with congenital abnormalities, immunodeficiency, metabolic disorders, chronic conditions, underlying lung diseases, airway foreign bodies, tuberculosis or nosocomial respiratory infections were excluded from this study.
Written informed consent was obtained from parents or legal guardians for all the subjects who are under 16. The study was approved by the Ethics Committee of Xinhua Hospital (approval number: Approval No. XHEC-D-2023-019), and conducted according to the principles in the Declaration of Helsinki.