BACKGROUND: The role of the antiviral therapy in treatment of COVID-19 is still a matter to be investigated. Also efficacy and safety of antiviral regimens were not compared according severity of the disease. In this study the efficacy and safety of hydroxychloroquine plus atazanavir/ritonavir was compared in patients with moderate and severe COVID-19. METHODS: We prospectively evaluated the clinical outcomes of 213 patients with COVID-19 during the hospitalization course and up to 56 days after the hospital discharge. The disease was categorized to moderate and severe based on the severity of pneumonia and peripheral oxygen saturation (SpO2). The patients received the national treatment protocol containing hydroxychloroquine (400 mg BD in first day and then 200 mg BD) plus atazanavir/ritonavir (300/100 mg daily) for 7 days. Main outcomes included discharge rates at day 7, 14 and 28, 28-day mortality, rate of intensive care unit (ICU) admission and intubation, length of hospital and ICU stay and incidence of adverse events. RESULTS: The mean (SD) age of patients was 60(14) years and 53% were male. According to WHO definition, 51.64% and 48.36% of the patients had moderate (SpO2 ≥ 90%) and severe disease (SpO2 < 90%) at baseline, respectively. The discharge rate of the moderate group was significantly higher than the severe group at day 7, 14 and 28 (HR = 0.49; 95% CI: 0.35-0.69, p = < 0.001 at day 7, HR = 0.48; 95% CI: 0.35-0.66, p = < 0.001 at day 14 and HR = 0.49; 95% CI: 0.36-0.67, p = < 0.001at day 28). The 28-day mortality of the severe group was six times higher than the moderate group (HR = 6.00; 95% CI: 2.50-14.44), p = < 0.001). The need of admission in ICU for the severe group and the moderate group was 37.86% and 18.18% of the patients. Length of hospital stay was significantly shorter in the moderate group in comparison with the severe group (5 ± 4 vs. 8 ± 6 days, p < 0.001). Patients in the moderate group experienced the serious adverse events and complications less than the severe group. The discharged patients were followed up to 56 days after discharge. Some of the patients complained of symptoms such as exertional dyspnea, weakness and new-onset hair loss. CONCLUSION: Our study did not support the use of hydroxychloroquine plus atazanavir/ritonavir in patients who had SpO2 < 90% at the time of hospital admission. SpO2 was the only predictor of clinical outcomes (duration of hospital stay, discharge from the hospital and mortality) in patients treated with hydroxychloroquine plus atazanavir/ritonavir.