Inclusion Criteria:  Patients who recently recovered from mild COVID-19 disease (First, second and third infection). Major organs function normally. Women at pregnant ages should be under contraception.. Willing and able to provide informed consent  Exclusion Criteria  Blood-borne infectious disease, e.g. hepatitis B.: History of mandatory custody because of psychosis or other psychological disease inappropriate for treatment deemed by treating physician. With other immune diseases, or chronic use of immunosuppressants or steroids. Compliance cannot be expected. Other conditions requiring exclusion deemed by physician