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.