A Study to Evaluate the Safety, Tolerability, and Efficacy of BIIB017 (Peginterferon Beta-1a) in Pediatric Participants for the Treatment of Relapsing-Remitting Multiple Sclerosis

This study will evaluate the safety, tolerability, and descriptive efficacy of BIIB017 in pediatric participants with relapsing-remitting multiple sclerosis (RRMS) and to assess the pharmacokinetics (PK) of BIIB017 in pediatric participants with RRMS in Part 1. In Part 2, the study will evaluate the long-term safety of BIIB017 and further describe safety and the long-term multiple sclerosis (MS) outcomes after BIIB017 treatment in participants who completed the study treatment at Week 96 in Part 1 of the study.

Key Inclusion Criteria: Part 1: * Must have a diagnosis of RRMS as defined by the revised consensus definition for pediatric MS. * Must have an EDSS score between 0.0 and 5.5. * Must have experienced \>= 1 relapse in the 12 months prior to randomization (Day 1) or \>= 2 relapses in the 24 months prior to randomization (Day 1) or have evidence of asymptomatic disease activity (Gd-enhancing lesions) on brain MRI in the 6 months prior to randomization (Day 1). Part 2: • Participants who completed the study treatment in Part 1 (Week 96 Visit), as per protocol. Key Exclusion Criteria: Part 1: * Primary progressive, secondary progressive, or progressive relapsing MS. These conditions require the presence of continuous clinical disease worsening over a period of at least 3 months. Participants with these conditions may also have superimposed relapses but are distinguished from relapsing participants by the lack of clinically stable periods or clinical improvement. * History of severe allergic or anaphylactic reactions or known drug hypersensitivity. * Known allergy to any component of Avonex or BIIB017 formulation. * Occurrence of an MS relapse that has occurred within 30 days prior to randomization (Day 1) and/or the participant has not stabilized from a previous relapse prior to randomization (Day 1). * Any previous treatment with PEGylated human IFN β-1a. Part 2: * Any significant changes in medical history occurring after enrollment in Part 1, including laboratory test abnormalities or current clinically significant conditions that, in the opinion of the Investigator, would have excluded the participant's participation in Part 1. The Investigator must re-assess the participant's medical fitness for participation and consider any factors that would preclude treatment. * The participant could not tolerate BIIB017 in Part 1. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.