MNX1 and lymphopenia: All positive (PCR confirmed) COVID-19 patients ≤65 years old who have Hb≤9 and at least one of the severe COVID-19 symptoms (tachypnea (breathing rate> 30 beats per minute), hypoxemia (O2 ≤93 saturation, the partial pressure ratio of arterial oxygen <300), Lung infiltration (> 50% of lung field within 24 to 48 hours), progressive lymphopenia, LDH>245 U/I, CRP>100) and are willing to cooperate in this project will be included in the study.