Material and methods
This study received ethical approval from the Tehran University of Medical Sciences, Tehran, Iran. All participants gave written informed consent, and the study was carried out following the Declaration of Helsinki.

Patient selection

Subjects of the study were children with confirmed SARS-CoV-2 pneumonia by real-time RT-PCR (admitted between 07 March and 10 June 2020) in Children’s Medical Center, an Iranian referral hospital who underwent the detection of peripheral blood lymphocyte subsets. The following information on each patient was extracted from electronic medical records: age, sex, underlying disease, intensive care unit (ICU) admission, need for invasive mechanical ventilation, laboratory findings, chest computed tomography (CT), and mortality. Chest CT scan was reported by an expert radiologist, and abnormal CT finding was considered if typical CT imaging features for COVID-19 including peripheral, bilateral, ground-glass opacification with or without consolidation or visible intralobular lines, multifocal ground-glass opacification or rounded morphology with or without consolidation or visible intralobular lines, or reverse halo sign or other findings of organizing pneumonia were reported (Simpson, Kay et al. 2020). On admission, severe illness was defined according to the following criteria: (1) breathing rate ≥30 times/min; (2) pulse oximeter oxygen saturation (SpO2) ≤93% at rest; and (3) ratio of the partial pressure of arterial oxygen (PaO2) to the fraction of inspired oxygen (FiO2) ≤300 mmHg (Wang, Nie et al. 2020).