Introduction: Cytokine storm and critical COVID-19 pneumonia are caused in at least 10% of patients by inborn errors of or auto-Abs to type I IFNs. The pathogenesis of life-threatening COVID-19 pneumonia in other patients remains unknown. Methods: This study was conducted at Masih Daneshvari Hospital, Tehran, Iran. In the period of study, 75 confirmed cases of COVID-19 with presentations ranging from mild upper respiratory tract infection to lower respiratory tract infection, including moderate, severe, and critical disease, were recruited. Expression of STING mRNA was measured in peripheral blood mononuclear cells (PBMCs) and compared between patients with different severity and outcome. Results: There was a significant negative correlation between age and STING expression level (p value = 0.010). Patients with “severe to critical” illness had a 20-fold lower STING expression level compared to the “mild to moderate” group (p value = 0.001). Also, the results showed lower expressions of STING in the patients admitted to the ICU (p value = 0.015). Patients who finally died had lower expression of STING at the time of sampling (p value = 0.041). Conclusion: STING mRNA expression in PBMCs was significantly lower in older COVID-19 cases, the patients with more severe illness, who needed intensive care, and who eventually died.