Inclusion Criteria:  Male or female participants, ages 4 to 17 years, inclusive Homozygous hemoglobin SS (HbSS) or hemoglobin S/beta0 thalassemia (HbS/β0 thal) Hemoglobin (Hb): Hb ≤10.5 g/dL at baseline Concomitant hydroxyurea (HU) therapy is allowed if the dose has been stable for at least 3 months with no anticipated need for dose adjustments during the study and no sign of hematological toxicity  Exclusion Criteria:  Any one of the following requiring medical attention within 14 days prior to signing the informed consent form (ICF):  Vaso-occlusive crisis (VOC) Acute chest syndrome (ACS) Splenic sequestration crisis Dactylitis Red blood cell (RBC) transfusion within 60 days of signing the ICF Evidence of abnormal high blood flow velocities on transcranial doppler (TCD) of 200 cm/sec or more Received an investigational drug within 30 days or 5 half-lives, whichever is longer, of signing the ICF