* 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).

* 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).

- 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).

- 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).