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 Multiple Sclerosis (RRMS) or onset of Secondary
Progressive Multiple Sclerosis (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 Multiple Sclerosis (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 Multiple Sclerosis (MS) drugs within 3 weeks or 5
times the half life (whichever is longer) prior to Day 1/Baseline

- Relapsing Remitting Multiple Sclerosis (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.