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

Estimated proportion of participants experiencing confirmed improvement in any 1 or more of the following components: a ≥1 point decrease in the Expanded Disability Status Scale (EDSS) score from a baseline score of <=6.0 (decrease sustained for ≥3 months); a ≥15% improvement from baseline in time to complete 9-Hole Peg Test (9HPT) by either hand (improvement sustained for ≥3 months for the same hand), where the time is the average time of 2 trials per hand at the same visit; a ≥15% improvement from baseline in time to complete Timed 25-Foot Walk (T25FW) test (improvement sustained for ≥3 months), where the time is the average time of 2 trials at the same visit; or a ≥15% improvement from baseline 3-Second Paced Auditory Serial Addition Test (PASAT-3) score (improvement sustained for 3 months or greater). Estimated proportion of responders is based on logistic regression adjusted for multiple sclerosis (MS) type, region and baseline component assessments.

Estimated proportion of participants experiencing confirmed clinical worsening in 1 or more components of the multicomponent endpoint (EDSS, T25FW, 9HPT, or PASAT-3) over 72 weeks, defined as: a ≥1.0 point increase in EDSS from a baseline score of ≤5.5 or a ≥0.5 point increase from a baseline score equal to 6.0 (increase sustained for 3 months or greater); a ≥15%worsening from baseline in time to complete T25FW test (worsening sustained for 3 months or greater), where the time is the average of 2 trials at the same visit; a ≥15% worsening from baseline in time to complete 9HPT by either hand (worsening sustained for 3 months or greater for the same hand), where the time is the average of 2 trials for each hand at the same visit; a ≥15% worsening from baseline in PASAT-3 score (worsening sustained for 3 months or greater). Estimated proportion of responders is based on logistic regression adjusted for MS type, region and baseline component assessments.

An AE was any untoward medical occurrence that did not necessarily have a causal relationship with this treatment. An SAE was any untoward medical occurrence that at any dose: resulted in death; in the view of the Investigators, placed the participant at immediate risk of death (a life-threatening event); however, this did not include an event that, had it occurred in a more severe form, might have caused death; required inpatient hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; resulted in a congenital anomaly/birth defect; any other medically important event that, in the opinion of the Investigators, could have jeopardized the participant or may have required intervention to prevent one of the other outcomes listed in the definition above.