Part 1: DLTs will be used to establish the RP2D. Part 2: The proportion of subjects per arm requiring artificial ventilation (intubation and mechanical ventilation [MV], extracorporeal membrane oxygenation [ECMO], heated, humidified high-flow nasal cannula oxygen [HFNC], noninvasive positive pressure ventilation [NiPPV]) or death from Day 1 through Day 29 of study treatment.

Part 1: DLTs will be used to establish the RP2D. Part 2: The proportion of subjects per arm requiring artificial ventilation (intubation and mechanical ventilation [MV], extracorporeal membrane oxygenation [ECMO], heated, humidified high-flow nasal cannula oxygen [HFNC], noninvasive positive pressure ventilation [NiPPV]) or death from Day 1 through Day 29 of study treatment.