ACE2 and COVID-19: The SARS-CoV-2 receptor binding domain (RBD) sS-protein have strong affinity with human ACE2, a property similar to other cororna virus (SARS COV) resembling to bat coronavirus HKU9-1.21 However, insufficient expression of ACE2 protein in children, combined with immature immune function and low immune response induced by the virus, children exibit less infetivity and mild clinical symptoms.22 According to the pathological studies of COVID-19 and SARS, a sharp decrease in T lymphocytes leads to the breakdown of the immune system.