CST-2 Phase II: To determine the ability of EIDD-2801 to reduce serious complications of COVID-19 including hospitalization, reduction in SAO2<92%, or death.;CST-2 Phase I: To determine the safety and tolerability of multiple ascending doses of EIDD-2801 to recommend dose for phase II.;CST6 Phase I: To determine the maximum safe dose of IV Favipiravir for efficacy evaluation in phase II;CST6 Phase I: To determine the safety and tolerability of multiple doses of IV Favipiravir in patients with COVID-19;Master Protocol: Dose-finding/Phase I;Master Protocol: Efficacy evaluation/Phase II - Mild to moderate patients (Group B);Master Protocol: Efficacy evaluation/Phase II - Severe patients (Group A)

CST-2 Phase II: To determine the ability of EIDD-2801 to reduce serious complications of COVID-19 including hospitalization, reduction in SAO2<92%, or death.;CST-2 Phase I: To determine the safety and tolerability of multiple ascending doses of EIDD-2801 to recommend dose for phase II.;CST6 Phase I: To determine the maximum safe dose of IV Favipiravir for efficacy evaluation in phase II;CST6 Phase I: To determine the safety and tolerability of multiple doses of IV Favipiravir in patients with COVID-19;Master Protocol: Dose-finding/Phase I;Master Protocol: Efficacy evaluation/Phase II - Mild to moderate patients (Group B);Master Protocol: Efficacy evaluation/Phase II - Severe patients (Group A)

CST-2 Phase II: To determine the ability of EIDD-2801 to reduce serious complications of COVID-19 including hospitalization, reduction in SAO2<92%, or death.;CST-2 Phase I: To determine the safety and tolerability of multiple ascending doses of EIDD-2801 to recommend dose for phase II.;Master Protocol: Dose-finding/Phase I;Master Protocol: Efficacy evaluation/Phase II - Mild to moderate patients (Group B);Master Protocol: Efficacy evaluation/Phase II - Severe patients (Group A)

CST-2 Phase II: To determine the ability of EIDD-2801 to reduce serious complications of COVID-19 including hospitalization, reduction in SAO2<92%, or death.;CST-2 Phase I: To determine the safety and tolerability of multiple ascending doses of EIDD-2801 to recommend dose for phase II.;Master Protocol: Dose-finding/Phase I;Master Protocol: Efficacy evaluation/Phase II - Mild to moderate patients (Group B);Master Protocol: Efficacy evaluation/Phase II - Severe patients (Group A)