A Study to Evaluate Efficacy, Safety, and Tolerability of EID of Natalizumab (BG00002) in Participants With RRMS Switching From Treatment With Natalizumab SID in Relation to Continued SID Treatment- Followed by Extension Study Comprising SC and IV Natalizumab Administration

Key Inclusion Criteria:

For Part 1:

   - Ability of the participant to understand the purpose and risks of the study and
   provide signed and dated informed consent and authorization to use confidential health
   information in accordance with national and local participant privacy regulations.

   - Diagnosis of relapsing remitting multiple sclerosis (RRMS) according to the McDonald
   criteria [Thompson 2018].

   - Treatment with natalizumab as disease-modifying monotherapy for RRMS that is
   consistent with the approved dosing for a minimum of 12 months prior to randomization.
   The participant must have received at least 11 doses of natalizumab in the 12 months
   prior to randomization with no missed doses in the 3 months prior to randomization.

   - Expanded Disability Status Scale (EDSS) score <=5.5 at screening.

   - No relapses in the last 12 months prior to randomization, as determined by the
   enrolling Investigator.

For Part 2:

   - Ability of the participants to understand the purpose and risks of the study and
   provide signed and dated informed consent for Part 2 and authorization to use
   confidential health information in accordance with national and local participant
   privacy regulations.

   - Completed Part 1 Week 72 visit while remaining on their randomized treatment
   assignment of SID or EID.

Key Exclusion Criteria:

For Part 1:

   - Primary and secondary progressive multiple sclerosis (MS).

   - MRI positive for Gd-enhancing lesions at screening.

   - Participants for whom MRI is contraindicated (e.g., have a contraindicated pacemaker
   or other contraindicated implanted metal device, have suffered, or are at risk for,
   side effects from Gd, or have claustrophobia that cannot be medically managed).

   - History of any clinically significant (as determined by the Investigator) cardiac,
   endocrinologic, hematologic, hepatic, immunologic, metabolic (including diabetes),
   urologic, pulmonary, neurologic (except for RRMS), dermatologic, psychiatric, renal,
   or other major disease that would preclude participation in a clinical study, in the
   opinion of the Investigator.

   - Presence of anti-natalizumab antibodies at screening.

For Part 2:

   - Participants treated with natalizumab EID was reverted to natalizumab SID by choice or
   as rescue treatment in Part 1.

   - Participant received treatment with any MS disease-modifying therapy other than
   natalizumab in Part 1 or in the period between Part 1 and Part 2.

   - History of human immunodeficiency virus or history of other immunodeficient
   conditions.

   - Current enrollment or a plan to enroll in any interventional clinical study in which
   an investigational treatment or approved therapy for investigational use is
   administered within 30 days (or 5 half-lives of the agent, whichever is longer) prior
   to the Baseline Visit or at any time during this study.

   - Inability to comply with study requirements.

   - Other unspecified reasons that, in the opinion of the Investigator or Biogen, make the
   participant unsuitable for enrollment.

The inclusion and exclusion criteria for new participants who did not participate in Part 1
of the study are the same as those for participants who did participate in Part 1.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply