Mechanical ventilation is defined as invasive mechanical ventilation (IMV) or extracorporeal membrane oxygenation (ECMO). Mechanical ventilation status was evaluated based on the National Institute of Allergy and Infectious Diseases (NIAID) clinical outcome 8-point ordinal scale. Participants whose clinical outcome met a NIAID score of 2 were considered as using mechanical ventilation.

The NIAID score is an 8-point ordinal scale of clinical outcomes: 1=death; 2=hospitalized, on invasive mechanical ventilation or ECMO; 3=hospitalized, on non-invasive ventilation or high flow oxygen devices; 4= hospitalized, requiring supplemental oxygen; 5=hospitalized, not requiring supplemental oxygen - requiring ongoing medical care (COVID 19 related or otherwise); 6=hospitalized, not requiring supplemental oxygen - no longer requires ongoing medical care; 7=not hospitalized, limitation on activities and/or requiring home oxygen; 8=not hospitalized, no limitations on activities.

Phase 2: Modified Intent-to-Treat (mITT) Analysis Set: All randomized participants who passed screening and received study drug (two enrolled and treated participants [1 each in the 6 mg/kg and placebo groups] were randomized and dosed in violation of inclusion/exclusion criteria and were thus excluded from this Set).

Mechanical ventilation is defined as invasive mechanical ventilation (IMV) or extracorporeal membrane oxygenation (ECMO). Mechanical ventilation status was evaluated based on the National Institute of Allergy and Infectious Diseases (NIAID) clinical outcome 8-point ordinal scale. Participants whose clinical outcome met a NIAID score of 2 were considered as using mechanical ventilation.

The NIAID score is an 8-point ordinal scale of clinical outcomes: 1=death; 2=hospitalized, on invasive mechanical ventilation or ECMO; 3=hospitalized, on non-invasive ventilation or high flow oxygen devices; 4= hospitalized, requiring supplemental oxygen; 5=hospitalized, not requiring supplemental oxygen - requiring ongoing medical care (COVID 19 related or otherwise); 6=hospitalized, not requiring supplemental oxygen - no longer requires ongoing medical care; 7=not hospitalized, limitation on activities and/or requiring home oxygen; 8=not hospitalized, no limitations on activities.

Phase 3: Intent-to-Treat (ITT) Analysis Set: All randomized participants.

Defined as the proportion of subjects with mechanical ventilation who have died by Day 29.

Phase 2: Modified Intent-to-Treat (mITT) Analysis Set: All randomized participants who passed screening and received study drug (two enrolled and treated participants [1 each in the 6 mg/kg and placebo groups] were randomized and dosed in violation of inclusion/exclusion criteria and were thus excluded from this Set).

Defined as the proportion of subjects with mechanical ventilation who have died by Day 29.

Phase 3: Intent-to-Treat (ITT) Analysis Set: All randomized participants.

Defined as time from randomization to a 2-point improvement on the NIAID 8-point ordinal scale, or discharge from the hospital, whichever came first. Participants who died before Day 29 were censored at Day 30. Kaplan-Meier method used to estimate the survival functions for each treatment arm.

The NIAID score is an 8-point ordinal scale of clinical outcomes: 1=death; 2=hospitalized, on invasive mechanical ventilation or ECMO; 3=hospitalized, on non-invasive ventilation or high flow oxygen devices; 4= hospitalized, requiring supplemental oxygen; 5=hospitalized, not requiring supplemental oxygen - requiring ongoing medical care (COVID 19 related or otherwise); 6=hospitalized, not requiring supplemental oxygen - no longer requires ongoing medical care; 7=not hospitalized, limitation on activities and/or requiring home oxygen; 8=not hospitalized, no limitations on activities.

Phase 2: Modified Intent-to-Treat (mITT) Analysis Set: All randomized participants who passed screening and received study drug (two enrolled and treated participants [1 each in the 6 mg/kg and placebo groups] were randomized and dosed in violation of inclusion/exclusion criteria and were thus excluded from this Set).

Defined as time from randomization to breathing room air (NIAID score ≥ 5), or discharge from the hospital, whichever came first. Participants who died before Day 29 were censored at Day 30.

Phase 2: Modified Intent-to-Treat (mITT) Analysis Set: All randomized participants who passed screening and received study drug (two enrolled and treated participants [1 each in the 6 mg/kg and placebo groups] were randomized and dosed in violation of inclusion/exclusion criteria and were thus excluded from this Set).

95% CI were calculated using Clopper-Pearson exact method based on the beta distribution.

Phase 2: Modified Intent-to-Treat (mITT) Analysis Set: All randomized participants who passed screening and received study drug (two enrolled and treated participants [1 each in the 6 mg/kg and placebo groups] were randomized and dosed in violation of inclusion/exclusion criteria and were thus excluded from this Set).

Defined as time from randomization to the date of a 1-point improvement on the NIAID score 8-point ordinal scale or discharge from the hospital, whichever occurred first, by Day 29. Participants who did not have 1-point improvement on the NIAID nor discharge from the hospital were censored at the date of the last NIAID 8-point ordinal scale assessment on/before Day 29. Participants who died were censored at Day 32.

Phase 2: Modified Intent-to-Treat (mITT) Analysis Set: All randomized participants who passed screening and received study drug.

Mortality rate at day 29 is the proportion of subjects who die by day 29. 95% CI were calculated using Clopper-Pearson exact method based on the beta distribution.

Phase 3: Intent-to-Treat (ITT) Analysis Set: All randomized participants.

Time to ventilation or death by Day 29 was defined as time (in days) from randomization to the date of death or start date of using mechanical ventilation (NIAID score ≤ 2) by Day 29. Participants who never had NIAID score ≤ 2 by Day 29 were censored at last assessment date of NIAID 8-point ordinal scale.

Phase 3: Intent-to-Treat (ITT) Analysis Set: All randomized participants.

Overall survival was defined as time from randomization to the date of death on/before Day 29. Participants who did not have a death record by Day 29 were censored at last date known alive on/before Day 29.

Phase 3: Intent-to-Treat (ITT) Analysis Set: All randomized participants.

Defined as time from randomization to the date of a 1-point improvement on the NIAID 8-point ordinal scale or discharge from the hospital, whichever occurred first, by Day 29. Participants who did not have 1-point improvement on the NIAID nor discharge from the hospital were censored at the date of the last NIAID 8-point ordinal scale assessment on/before Day 29. Participants who died were censored at Day 35.

Phase 3: Intent-to-Treat (ITT) Analysis Set: All randomized participants.