A Study to Determine the Efficacy and Safety of Luspatercept in Adult Participants and to Evaluate the Safety and Pharmacokinetics in and Adolescent Participants With Alpha (α)-Thalassemia

Inclusion Criteria: * Adult participant≥ 18 years with documented diagnosis of A-Thal HbH disease with Transfusion dependence defined as:. i) TD participant: ≥ 6 RBC units during the 24 weeks prior to randomization. ii) NTD participant:\< 6 RBC units during the 24 weeks prior to randomization(transfusion due to conditions other than A-Thal will not be considered)and, RBC transfusion-free during at least 8 weeks prior to randomization(unless transfusion was required to treat an acute medical condition other than A-Thal) and, mean baseline Hb ≤ 10 g/dL, based on a minimum of 2 measurements ≥ 1 week apart within 4 weeks prior to randomization; hemoglobin values within 21 days post-transfusion will be excluded. * Adult participant has Eastern Cooperative Oncology Group (ECOG) 34 score of 0 or 1. * Adolescent participant 12 years to \< 18 years with documented diagnosis of A-Thal HbH disease with transfusion dependence defined as:. i) TD participant: ≥ 4 RBC events during the 24 weeks prior to enrollment and, no transfusion-free period for \> 56 days during the 24 weeks prior to enrollment. Participants must have a history of regular transfusions for at least 2 years. ii) NTD participant:\< 4 RBC events during the 24 weeks prior to enrollment and RBC transfusion-free during at least 8 weeks prior to enrollment and, mean baseline Hb ≤ 10 g/dL, based on a minimum of 2 measurements ≥ 1 week apart within 4 weeks prior to enrollment, hemoglobin values within 21 days post-transfusion will be excluded. iii) Participant has Karnofsky (age ≥16 years) or Lansky (age \< 16 years) performance status score ≥ 50 at screening. Exclusion Criteria: * Medical Conditions: Diagnosis of A-ThalTrait, Hb Bart hydrops, ATRx A-Thal, hemoglobin S/β-thalassemia, myelodysplasia subtype anemia, or with HbE homozygous beta gene mutation. Anemia related to nutritional deficiency, anemia of chronic disease, autoimmune hemolytic anemia, or any other hemolytic anemias. Undergone episodes of hemolysis not related to A-Thal within the 8 weeks prior to randomization. Applicable for the EU only: Bleeding disorders manifested by frequent bleeding episodes (eg, menorrhagia, epistaxis, clotting disorders). * Participant has deep vein thrombosis (DVT), stroke or other thromboembolic event(s) (except clogged indwelling catheter) requiring medical intervention ≤ 24weeks prior to randomization. * Participant has uncontrolled hypertension. Controlled hypertension for this protocol is considered: blood pressure value corresponding to ≤Grade 1 according to NCI CTCAE Version 5.0. with or without pharmacological treatment. * Reproductive Status: Women who are pregnant, plan to get pregnant during the study, or who are breastfeeding. * Prior/Concomitant: Undergone HSCTs or gene therapy (candidates for HSCT or gene therapy with waiting period of ≥ 12 months are eligible). * Use of hydroxyurea treatment ≤ 12 weeks prior to enrollment for NTD participants and ≤ 24 weeks for TD participants. * Participant who has extramedullary hematopoiesis (EMH) complications requiring treatment to control the growth of EMH mass(es) during the screening period. * Any medical or psychiatric condition (including active infections, recent surgery, sequelae of diseases or interventions, clinically significant laboratory abnormalities or concurrent treatment) that in the opinion of the investigator would put the participant at unacceptable risk of participating in the study or that could affect interpretability of data. * Other protocol-defined inclusion/exclusion criteria apply.