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

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 …