A Study to Investigate the Efficacy, Pharmacokinetics, and Safety of Mitapivat in Pediatric Participants With Transfusion-Dependent Alpha- or Beta-Thalassemia (α- or β-TDT)

Inclusion Criteria: * Written informed consent/assent from the participant (or their legally authorized representative, parent(s), or legal guardian) must be obtained before any study-related procedures are conducted and participants must be willing to comply with all study procedures for the duration of the study. * Aged 1 to \<18 years and weighing at least 7 kilograms (kg) at the time of providing informed consent/assent. * Documented diagnosis of thalassemia (β-thalassemia with or without α-globin gene mutations, hemoglobin E (HbE)/β-thalassemia, or α-thalassemia/hemoglobin H (HbH) disease) based on Hemoglobin (Hb) electrophoresis, Hb high-performance liquid chromatography, and/or DNA analysis from the participant's medical record. If this information is not available from the participant's medical record, the test(s) can be performed by a local laboratory during the Screening Period. If a local laboratory is unable to perform the test(s), results from the comprehensive α- and β-globin genotyping performed by the study central laboratory can be used. * Transfusion dependent, defined as 6 to 20 transfusion episodes (also referred to as "transfusion events") and a ≤6-week transfusion-free period during the 24-week period before randomization. * If taking hydroxyurea, the hydroxyurea dose must be stable for ≥16 weeks before randomization. * Female participants who have attained menarche must be abstinent of sexual activities that may induce pregnancy as part of their usual …