Part 1: The primary objective is to evaluate the efficacy of natalizumab extended interval dosing (EID) (every 6 weeks [Q6W]) in participants who have previously been treated with natalizumab standard interval dosing (SID) (every 4 weeks [Q4W]) for at least 12 months, in relation to continued Q4W treatment. The secondary objectives is to evaluate relapse-based clinical efficacy measures, disability worsening, additional Magnetic resonance imaging (MRI)-lesion efficacy measures and safety of Q6W in participants who have previously been treated with natalizumab Q4W for at least 12 months, in relation to continued Q4W treatment. Part 2: The primary objective is to evaluate participant preference for subcutaneous (SC) versus intravenous (IV) route of natalizumab administration. The secondary objectives is to evaluate treatment satisfaction, drug preparation and administration time, safety and immunogenicity, efficacy and characterize pharmacokinetic (PK) and pharmacodynamic (PD) drug preparation and administration time of SC versus IV routes of natalizumab administration.