Efficacy is measured by a composite endpoint of proportion of patients who die, develop respiratory failure \[require mechanical ventilation\], or require intensive care unit \[ICU\] care for the treatment of COVID-19. Analyses are cumulative, thus analysis on Day 29 includes all events till that day. Patients who developed respiratory failure and/or required ICU at randomization are excluded from the analysis.

Event Free Survival

Overall response rate at Day 28 after randomization was defined as the percentage participants in each arm demonstrating a complete response (CR) or partial response (PR), based on investigator assessment \& according to standard criteria, without requirement for additional systemic therapies for an earlier progression, mixed response or non-response. Scoring of response was relative to the organ stage at the time of randomization. CR was defined as a score of 0 for the aGvHD grading in all evaluable organs that indicates complete resolution of all signs \& symptoms of aGvHD in all evaluable organs without administration of additional systemic therapies for any earlier progression, mixed response or non-response of aGvHD. PR was defined as improvement of 1 stage in 1 or more organs involved with aGvHD signs or symptoms without progression in other organs or sites without administration of additional systemic therapies for an earlier progression, mixed response or non-response of aGvHD.

Summary of adverse events (all grades).

Proportion of patients achieving Hct control at Week 28 was defined by the absence of phlebotomy eligibility starting at Week 8 and continuing through Week 28, with no more than one phlebotomy eligibility occurring post randomization and prior to Week 8. Phlebotomy eligibility was defined by: * Confirmed Hct \> 45% that is at least 3 percentage points higher than the Hct obtained at Baseline Or * Confirmed Hct \> 48% The confirmation occurred 2 to 14 days subsequent to the initial observation.

The change in spleen volume from baseline to week 48 was measured by magnetic resonance imaging (MRI) (or by computer tomography (CT) for participants unable to undergo MRI) and was calculated only for participants who had an evaluable spleen volume at baseline. The percentage of participants achieving a greater than or equal to 35% reduction in spleen volume from baseline to week 48 was then calculated by treatment group.

Percentage of participants achieving a 50% reduction in spleen length at week 24. For subjects with palpable spleen at baseline and non-palpable at post-baseline, the post-baseline spleen are imputed as 0. Subjects who had palpable, but missing spleen length at baseline is excluded from the analysis. Subjects with missing spleen length at Week 24 or who withdraw earlier from the study are considered as a non-responder. The 95% CI is computed using exact Clopper-Pearson method.

Change of RBC transfusion requirement measured as percent change of the hematocrit-adjusted volume of transfused RBC and observed during within on-treatment interval (any time-points of RBC transfusion between week 6 and week 30 driven by the individual patient's need) compared to baseline (defined by pre-treatment interval between Week - 24 to start of treatment).

The primary measure of spleen size was by MRI. MRIs were performed with a body coil because the objective was to measure organ volume only, not to assess for lesions. MRIs were performed by local radiologists who were instructed not to provide a quantitative measure of spleen volume, but could provide a qualitative assessment such as enlarged, smaller, larger, etc. The scans from an individual patient were to be read by a central reader upon transfer from the site radiologist.

Measurement in acute GvHD and SR-acute GvHD subjects used extensive PK sampling in Groups 1-3 sparse sampling in Group 4. AUClast: The AUC from time zero to the last measurable concentration sampling time (Tlast).

Measurement in acute GvHD and SR-acute GvHD subjects used extensive PK sampling in Groups 1-3 and sparse sampling in Group 4. Cmax: The maximum (peak) observed plasma drug concentration

Measurement in acute GvHD and SR-acute GvHD subjects used extensive PK sampling in Groups 1-3 and sparse sampling in Group 4. T1/2: The elimination half-life associated with the terminal slope (Lambda\_z ) of a semi logarithmic concentration-time curve

Measurement in acute GvHD and SR-acute GvHD subjects used extensive PK sampling in Groups 1-3 and sparse sampling in Group 4. Ctrough: The minimum observed plasma concentration at the end of an administration interval (corresponding to the pre-dose concentration prior to the following administration).

Phase I: Age-based determination of RP2D in Groups 2 and 3 and was based on observed PK parameters in Groups 1-3 * Group 2: age ≥ 6 to \< 12 years * Group 3: age ≥ 2 to \< 6 years AUClast: The AUC from time zero to the last measurable concentration sampling time (Tlast).

Phase I: Age-based determination of RP2D in Groups 2 and 3 and was based on observed PK parameters in Groups 1-3 * Group 2: age ≥ 6 to \< 12 years * Group 3: age ≥ 2 to \< 6 years Cmax: The maximum (peak) observed plasma drug concentration.

Phase I: Age-based determination of RP2D in Groups 2 and 3 and was based on observed PK parameters in Groups 1-3 * Group 2: age ≥ 6 to \< 12 years * Group 3: age ≥ 2 to \< 6 years Ctrough: The minimum observed plasma concentration at the end of an administration interval (corresponding to the pre-dose concentration prior to the following administration).

Phase II: ORR is defined as the percentage of patients demonstrating a complete response (CR) or partial response (PR) without requirement for additional systemic therapies for an earlier progression, mixed response or non-response. Scoring of response was relative to the organ stage at the start of the study treatment. Complete response (CR) is defined as a score of 0 for the aGvHD grading in all evaluable organs that indicates complete resolution of all signs and symptoms of aGvHD in all evaluable organs without administration of additional systemic therapy for any earlier progression, mixed response or non-response of aGvHD. Partial response (PR) is defined as improvement of 1 stage in 1 or more organs involved with aGvHD signs or symptoms without progression in other organs or sites without administration of additional systemic therapy for an earlier progression, mixed response or non-response of aGvHD.