During a 1 month screening period (month -1), inclusion and exclusion criteria will be verified, and subjects will be enrolled in the GA only portion following registration with the web-based data entry system and at month 3, they will be randomly assigned to treatment that will be stratified according to study site in varying block sizes One day after the last natalizumab infusion (month 0), 200 Patients on natalizumab therapy will be started on GA. On day 1 of month 3, subjects will be randomized 1:1 to receive either: