Examination of Related Factor to Favipiravir Plasma Concentrations and Clinical Efficacy in COVID-19

Effective antiviral agents were unavailable in the initial stages of the coronavirus disease 2019 (COVID-19) pandemic. Favipiravir (FPV) was expected to be a therapeutic drug. Despite the higher EC50 for severe acute respiratory syndrome coronavirus 2 compared to that for the influenza virus and the high variability in the FPV plasma concentrations, the FPV dosage was determined based on that for influenza. We explored the relationship between FPV trough concentrations and efficacy and evaluated factors affecting the FPV trough plasma concentrations.