All study participants in 101-MS-322 (NCT00306592) and 101-MS-321 (NCT00297232) received open label 300 mg intravenous (IV) natalizumab 60-minute infusion once every 4 weeks (28 days ±7 days) for up to 48 weeks. After 48 weeks, participants from 101-MS-322 (NCT00306592) entering study 101-MS-321 (NCT 00297232; considered the Long-Term Treatment Period of 101-MS-322) were continued on treatment from Week 52 through Week 480.

AEs: any sign, symptom, or diagnosis/disease that is unfavorable or unintended, that is new, or if pre-existing, worsens in participants administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. SAEs: an event that results in death; an event that, in the view of the investigator, places the participant at immediate risk of death (a life-threatening event); an outcome that results in a congenital anomaly/birth defect diagnosed in a child of a participant; an event that requires or prolongs inpatient hospitalization; an event that results in persistent or significant disability/incapacity. Any other medically important event that, in the opinion of the investigator, may jeopardize the participant or may require intervention to prevent one of the other outcomes listed in the definition above. Treatment-emergent AEs: events in participants who had received at least 1 dose of study drug, regardless of relationship to study drug.

For purposes of this analysis, the terms 'hypersensitivity' and 'drug hypersensitivity' were categorized by their temporal relationship to study drug infusion (within 2 hours of the start of the infusion), and were considered equivalent. Hypersensitivity reactions are defined as infusion reactions with the following preferred terms: hypersensitivity not otherwise specified (NOS), anaphylactic reaction, anaphylactoid reaction, dermatitis allergic, drug hypersensitivity, urticaria NOS, vasoconstriction, urticaria generalised, hypersensitivity, urticaria.

'Positive with unknown persistence' is defined as a positive result (≥0.5 micrograms/mL) at one timepoint only with no confirmatory re-test available at least 42 days later. 'Transient positive' is defined as a positive at one timepoint but negative upon re-test at least 42 days later. 'Persistent positive' is defined as positive at 2 or more timepoints separated by at least 42 days. The threshold for classifying a sample as 'antibody positive' was set at the lowest level of reactivity that had a measurable impact on drug serum concentrations.