Study to Assess the Efficacy, Safety, Tolerability, and Pharmacokinetics of BIIB033 in Participants With Relapsing Forms of Multiple Sclerosis When Used Concurrently With Avonex

Key Inclusion Criteria: * Diagnosis of relapsing remitting MS (RRMS) or onset of secondary progressive MS (SPMS) * RRMS and SPMS subjects must have evidence of ongoing disease activity within 12 months of enrollment. * All male and female subjects of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception for at least 6 months after their last dose of study treatment Key Exclusion Criteria: * A MS relapse that has occurred within the 90 days prior to Day 1/Baseline and/or the subject has not stabilized from a previous relapse prior to Screening * Previous history of clinically significant disease. * Plans to undergo elective major procedures/surgeries at any time during the study. * Treatment with any investigational MS drugs within 3 weeks or 5 times the half life (whichever is longer) prior to Day 1/Baseline * RRMS subjects with any history of inadequate response to any approved interferon β preparation * History of human immunodeficiency virus (HIV), hepatitis C virus antibody, or hepatitis B virus * History or evidence of drug or alcohol abuse within 2 years prior to randomization Note: Other protocol defined inclusion/exclusion criteria may apply.