Inclusion Criteria:  the patients diagnosed as the common or severe cases of COVID-19 aged 18-85 the absolute lymphocyte counts below the normal value (<1.1-3.2×109/L)  Exclusion Criteria:  the patients who are diagnosed as critically ill cases or participating in other clinical trials women who are pregnant or lactating ALT/AST > 5 times upper limit of normal (ULN), neutrophils counts < 0.5×109/L, platelets counts< 50×109/L patients diagnosed with rheumatoid immune-related diseases patients who take long-term oral anti-rejection drugs or immunomodulatory drugs hypersensitive reaction to nicotinamide or any auxiliary materials patients with active tuberculosis or combined with bacterial and fungal infections patients undergoing organ transplant patients with mental disorders.