Key Inclusion Criteria:

   - Any age at the day of screening visit.

   - No known or suspected clinically significant disease from AdV, BKV, CMV, EBV, HHV-6,
   and/or JCV

   - Within 25 days of receiving a first allogeneic HCT and have demonstrated clinical
   engraftment at time of dosing

   - Meet one or more of the following criteria at the time of randomization:

      - Related (sibling) donor with at least one mismatch at one of these HLA-gene loci:
      HLA-A, -B or -DR

      - Haploidentical donor

      - Matched or Mismatched unrelated donor

      - Use of umbilical cord blood as stem cell source

      - Ex vivo graft manipulation resulting in T cell depletion

      - Received anti-thymocyte globulin or alemtuzumab (Campath-1H)

Key Exclusion Criteria:

   - History of AdV, BKV, CMV, EBV, HHV-6, and/or JCV end-organ disease within 6 months
   prior to randomization

   - Evidence of active Grade >2 acute GVHD

   - Presence of non-minor uncontrolled or progressive bacterial, viral or fungal
   infections

   - Known history or current (suspected) diagnosis of Grade ≥3 CRS requiring treatment
   associated with the administration of peptides, proteins, and/or antibodies

   - Ongoing therapy with high-dose systemic corticosteroids (ie, prednisone equivalent
   dose >1.0 mg/kg/day) within 24 hours prior to dosing

   - Relapse of primary malignancy other than minimal residual disease

Note: Other protocol defined Inclusion/Exclusion criteria may apply.