A Study to Investigate the Efficacy, Pharmacokinetics, and Safety of Mitapivat in Pediatric Participants With α- or β-Non-Transfusion-Dependent Thalassemia

The primary objective of this study is to compare the effect of mitapivat versus placebo on anemia in pediatric participants with alpha- or beta-non-transfusion-dependent thalassemia.

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 \2.5\*Upper Limit of Normal (ULN) (unless due to hemolysis and hepatic iron deposition) and Alanine Transaminase (ALT) \>2.5\*ULN (unless due to hepatic iron deposition). * Renal dysfunction as defined by an estimated glomerular filtration rate \215 milligrams per deciliter (mg/dL) \[5 millimole per liter (mmol/L)\]. * Active infection requiring systemic antimicrobial therapy at the time of providing informed consent/assent. If antimicrobial therapy is required during the Screening Period, screening procedures should not be performed while antimicrobial therapy is being administered, and the last dose of antimicrobial therapy must be administered ≥7 days before randomization. * Participants with known active hepatitis B or hepatitis C virus infection. * Participants with known human immunodeficiency virus (HIV) infection. * History of major surgery (including splenectomy) ≤16 weeks before providing informed consent/assent and/or a major surgical procedure planned during the study. * Current enrollment or past participation (within ≤12 weeks or a timeframe equivalent to 5 half-lives of the investigational study drug before administration of the first dose of study drug, whichever is longer) in any other clinical study involving an investigational treatment or device. * Receiving strong Cytochrome3A4/5 (CYP3A4/5) inhibitors that have not been stopped for ≥5 days or a timeframe equivalent to 5 half-lives (whichever is longer), or strong CYP3A4/5 inducers that have not been stopped for ≥4 weeks or a timeframe equivalent to 5 half-lives (whichever is longer), before randomization. * Receiving anabolic steroids that have not been stopped for at least 4 weeks before randomization. Testosterone replacement therapy to treat hypogonadism is allowed; the testosterone dose and preparation must be stable for ≥12 weeks before randomization. * Known allergy, or other contraindication, to mitapivat or its excipients (microcrystalline cellulose, croscarmellose sodium, sodium stearyl fumarate, mannitol, magnesium stearate, and the Opadry filmcoat \[hypromellose, titanium dioxide, lactose monohydrate triacetin, and FD\&C Blue #2\]). * Any medical, hematological, psychological, or behavioral condition(s) or prior or current therapy that, in the opinion of the Investigator, may confer an unacceptable risk to participating in the study and/or could confound the interpretation of the study data. Also excluded are: * Participants who are institutionalized by regulatory or court order. * Participants with any condition(s) that could create undue influence (including but not limited to incarceration, involuntary psychiatric confinement, and financial or familial affiliation with the Investigator or Sponsor).