Unrelated Donor Transplant Versus Immune Therapy in Pediatric Severe Aplastic Anemia

Inclusion Criteria: 1. Confirmed diagnosis of idiopathic SAA, defined as: * Bone marrow cellularity \<25%, or \<30% hematopoietic cells. * Two out of three of the following (in peripheral blood): neutrophils \<0.5 x109/L, platelets \<20 x109/L, reticulocyte count \<60 x109/L with hemoglobin \<8g/dL. 2. Age ≤25 years old. 3. No suitable fully matched related donor available (minimum 6/6 match for Human Leukocyte antigen (HLA) -A and B at intermediate or high resolution and DRB1 at high resolution using DNA based typing). 4. At least two unrelated donors noted on National Marrow Donor Program (NMDP) search who are well matched (9/10 or 10/10 for HLA-A, B, C, DRB1, and DQB1 using high resolution). 5. Signed informed consent for the randomized trial by patient and/or legal guardian. 6. Adequate organ function defined as in the judgment of the investigator, there is not irreversible organ damage that would preclude the patient from meeting the organ function inclusion criteria for HSCT listed in section 2.3.4 by the intended time of HSCT (6-8 weeks after randomization) or preclude patients from receiving horse ATG. Exclusion Criteria: 1. Inherited bone marrow failure syndromes (IBMFS). The diagnosis of Fanconi anemia must be excluded by diepoxybutane (DEB) or equivalent testing on peripheral blood or marrow. Telomere length testing should be sent on all patients to exclude Dyskeratosis congenita, but if results are delayed or unavailable and there are no clinical manifestations of DC, patients may enroll. If patients have clinical characteristics suspicious for Shwachman Diamond syndrome, this syndrome must be excluded by pancreatic isoamylase testing or gene mutation analysis. Note: pancreatic isoamylase testing is not accurate in children less than 3 years. 2. Clonal cytogenetic abnormalities or fluorescence In Situ Hybridization (FISH) pattern consistent with pre-myelodysplastic syndrome (pre-MDS) or MDS on marrow examination (see section 4.2.3.1 for details of the required MDS FISH panel). 3. Known severe allergy to horse ATG. 4. Prior allogeneic stem cell transplant. 5. Prior solid organ transplant. 6. Infection with human immunodeficiency virus (HIV). 7. Active Hepatitis B or C. This should be excluded in patients where there is clinical suspicion of hepatitis (e.g. elevated LFTs). 8. Female patients who are pregnant or breast-feeding. 9. Prior malignancies except resected basal cell carcinoma or treated cervical carcinoma in situ.