The mechanism of peripheral blood lymphocyte decline in COVID-19 patients is not yet clear. However, one theory demonstrated that in the whole progression of COVID-19, the extensive activation of poly-ADP-ribose polymerase (PARP) may reduce the cellular NAD+ and dampen the adaptive immune system. So investigators presume that replenishing the NAD+ using nicotinamide as the precursor may improve the lymphocyte counts and boost the adaptive immune system. As a result, the study using nicotinamide as a kind of supportive therapy provide further evidence of their efficiency and safety in treating lymphopenia in patients with COVID-19.

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.