Among the hematological parameters, Hb, Ht, and RBC were significantly lowered, and MCV increased in COVID-19 patients before hospitalization compared to healthy subjects (p < 0.001, p = 0.028, p < 0.001, p = 0.001, respectively), while after hospitalization of these patients, the MCV value and platelet count increased (p = 0.041, p = 0.002, respectively), and the white blood cell count significantly decreased (p = 0.047). The gene discussed is GSTM1; the disease is COVID-19.