Inclusion Criteria:  Signed informed consent. Requiring admission to hospital for acute medical care (not for purely public health or quarantine purposes). Current respiratory failure (i.e. receipt of high-flow nasal cannula, non-invasive ventilation, invasive mechanical ventilation, or ECMO (extracorporeal membrane oxygenation) used to treat acute hypoxemic respiratory failure). SARS-CoV-2 (COVID-19) infection, documented by a nucleic acid test (NAT) or equivalent testing with most recent rest within 14 days prior to randomization. Respiratory failure is believed to be due to SARS-CoV-2 pneumonia.  Exclusion Criteria:  Known allergy to investigational agent or vehicle. More than 4 days since initiation of support for respiratory failure. Chronic/home mechanical ventilation (invasive or non-invasive) for chronic lung or neuromuscular disease (non-invasive ventilation used solely for sleep-disordered breathing is not an exclusion). Moribund patient (i.e. not expected to survive 24 hours). Active use of "comfort care" or other hospice-equivalent standard of care. Expected inability to participate in study procedures. In the opinion of the investigator, any condition for which, participation would not be in the best interest of the participant or that could limit protocol-specified assessments. Previous enrollment in TESICO  Agent-specific exclusion criteria  Prior receipt of any dose of remdesivir during present illness (remdesivir agent). GFR (glomerular filtration rate) < 30 ml/min and not receiving dialysis (remdesivir agent). ALT (alanine aminotransferase) or AST (aspartate aminotransferase) > 10 times upper limit of normal (remdesivir agent). Unwillingness to commit to avoid sex that may result in pregnancy for at least 7 days after completion of remdesivir vs. placebo (remdesivir agent). Refractory hypotension (aviptadil agent). Severe diarrhea (Aviptadil agent). Current C. difficile infection (aviptadil agent). Pregnancy or current breast-feeding (aviptadil agent). End-stage liver disease (aviptadil agent).