Participants who are already on natalizumab treatment, 300 milligrams (mg) IV infusion and who decide to switch to 2x150 mg SC injection administered as standard of care/routine clinical practice will be observed for up to 12 months.

Participants who initiate natalizumab, 300 mg, IV infusion injection administered as standard of care/routine clinical practice will be observed for up to 12 months

Participants who initiate natalizumab, 2x150 mg, SC injection administered as standard of care/routine clinical practice will be observed for up to 12 months.

Administered as specified in the treatment arm.

The participant preference will be measured by Patient preference questionnaire (PPQ) 1. PPQ 1 comprises of 3 questions - 1. "Are you satisfied with the route of administration of natalizumab?" (yes/no) and indicate main reason. 2. For SC participants only- "Have you experienced adverse events related to a subcutaneous injection." (1= mild to 5 = severe), and 3. "If you had to choose between subcutaneous or intravenous route again, which route would you choose?".

The participant preference will be measured by Patient preference questionnaire (PPQ) 1. PPQ 1 comprises of 3 questions - 1. "Are you satisfied with the route of administration of natalizumab?" (yes/no) and indicate main reason. 2. For SC participants only- "Have you experienced adverse events related to a subcutaneous injection." (1= mild to 5 = severe), and 3. "If you had to choose between subcutaneous or intravenous route again, which route would you choose?".