Severe fever with thrombocytopenia syndrome (SFTS) is a disease caused by SFTS virus (SFTSV), an RNA virus, and endemic to Asia. The case fatality rate in Japan is estimated to be approximately 27%. Currently, no efficacious drugs are available. Favipiravir (FPV) has the potential to treat SFTS. However, no quantitative studies comparing FPV with conventional treatments have been conducted in the country. We conducted an open-label, interventional study in which patients with SFTS were administered FPV 1800 mg twice on the first day, followed by 800 mg twice daily for nine days. Additionally, we performed a retrospective observational study on patients with SFTS who received the best supportive care (BSC) without FPV treatment by collecting medical data from individuals at the same institutions. Patients aged 20-85 years were recruited, and blood samples were collected on Days 0 (pre-dose), 1, 3, 6, 9, 14, and 27 in the interventional study. Outcomes and laboratory parameters were compared by 1:1 matching using propensity score (PS). Thirty and 78 patients were enrolled in the FPV and the BSC groups, respectively. Twenty-three pairs were identified by PS matching. The risk ratio of fatality for the FPV group compared to the BSC group reduced to 0.500 after matching. Ferritin levels associated with hemophagocytosis severity were significantly different between the FPV and BSC groups on Days 3 to 9 after matching. No concerning serious adverse events were observed in the FPV group.This interventional study has been registered with the Japan Registry of Clinical Trials as jRCT2080223816
