A Study to Investigate Efficacy and Safety of Teplizumab Compared With Placebo in Participants 1 to 25 Years of Age With Stage 3 Type 1 Diabetes

This is a multicenter, randomized, double-blind, parallel, placebo-controlled Phase 3, 2-arm study for treatment. The purpose of this study is to measure change in glycemic control and prandial insulin independency over 52 weeks with teplizumab compared with placebo, both administered by intravenous (IV) infusion, in participants with recently diagnosed Stage 3 type 1 diabetes (T1D) aged 1 to 25 years, on standard insulin therapy.

Inclusion Criteria: * Participants are eligible to be included in the study only if all of the following criteria apply: * Participant must be 1 to 25 years of age inclusive, at the time of signing the informed consent. * Participants diagnosed with T1D Stage 3 according to American Diabetes Association 2025 criteria * Participants able to be randomized and initiate study drug within 8 weeks (56 days) of the Stage 3 T1D diagnosis * Participants must be positive for at least one T1D autoantibody at screening: * Glutamic acid decarboxylase (GAD-65), * Insulinoma Antigen-2 (IA-2), * Zinc-transporter 8 (ZnT8), or * Insulin (if obtained not later than 14 days after exogenous insulin therapy initiation). * Islet cell cytoplasmic autoantibodies (ICAs) * Have random C-peptide level ≥0.2 nmol/L obtained at screening * Enter Inclusion Criteria Sex * Both male and female participants are eligible. * Contraceptive use by women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. * A female participant is eligible to participate if she is not pregnant, and one of the following conditions applies: * Is a woman of nonchildbearing potential (WONCBP) OR * Is a woman of childbearing potential (WOCBP) and agrees to use a contraceptive method that is highly effective, with a failure rate of \