Study of Efficacy and Safety of Ruxolitinib in Patients With Grade II to IV Steroid-refractory Acute Graft vs. Host Disease

Key Inclusion criteria * Male or female Chinese participants aged 12 or older at the time of informed consent. Written informed consent from participant, parent or legal guardian. * Able to swallow tablets. * Have undergone alloSCT from any donor source (matched unrelated donor, sibling, haplo-identical) using bone marrow, peripheral blood stem cells, or cord blood. * Clinically diagnosed Grades II to IV acute GvHD as per standard criteria occurring after alloSCT requiring systemic immune suppressive therapy. * Evident myeloid and platelet engraftment (confirmed within 48 hours prior to study treatment (ruxolitinib) start): * Confirmed diagnosis of steroid refractory aGvHD defined as participants administered systemic corticosteroids (methylprednisolone at least 1 mg/kg/day \[or equivalent prednisone dose at least 1.25 mg/kg/day\]), given alone or combined with calcineurin inhibitors (CNI) and either: 1. Progression based on organ assessment after at least 3 days compared to organ stage at the time of initiation of systemic corticosteroid +/- CNI for the treatment of Grade II to IV aGvHD. OR 2. Failure to achieve at a minimum partial response based on organ assessment after 7 days compared to organ stage at the time of initiation of systemic corticosteroid +/-CNI for the treatment of Grade II to IV. OR 3. Participants who fail corticosteroid taper defined as fulfilling either one of the following criteria: * Requirement for an increase in the corticosteroid dose to methylprednisolone ≥ 1 mg/kg/day (or equivalent prednisone dose ≥ 1.25 mg/kg/day). OR * Failure to taper the methylprednisolone dose to \< 0.5 mg/kg/day (or equivalent prednisone dose \<0.6 mg/kg/day) for a minimum of 7 days. Key Exclusion criteria * Has received more than one systemic treatment for steroid refractory aGvHD. Participants who received JAK inhibitor therapy for any indication after initiation of current alloSCT conditioning. * Clinical presentation resembling de novo chronic GvHD or GvHD overlap syndrome with both acute and chronic GvHD features. * Failed prior alloSCT within the past 6 months. Presence of relapsed primary malignancy after the alloSCT was performed. * Presence of an active uncontrolled infection including significant bacterial, fungal, viral or parasitic infection requiring treatment. * SR-aGvHD occurring after non-scheduled donor lymphocyte infusion (DLI) administered for pre-emptive treatment of malignancy recurrence. Note: Participants who have received a scheduled DLI as part of their transplant procedure and not for management of malignancy relapse are eligible. * Presence of significant respiratory disease, severely impaired renal function, clinically significant or uncontrolled cardiac disease, unresolved cholestatic and liver disorders (not attributable to aGvHD). Disorders and/or current therapy with medications that interfere with coagulation or platelet function. Other protocol-defined inclusion / exclusion criteria may apply