The primary objective of this study is to assess the effect of natalizumab compared to fingolimod on the evolution of new on-treatment T1-gadolinium-enhancing (Gd+) lesions to persistent black holes (PBH) over 52 weeks. The secondary objectives of this study in this study population are to assess the effect of natalizumab compared to fingolimod on: magnetic resonance imaging (MRI) measures of central nervous system (CNS) tissue destruction as measured by the number of new T1-Gd+ lesions; various other MRI measures of disease activity; No Evidence of Disease Activity (NEDA); Relapse on treatment over 52 weeks; The change in information processing speed as measured by the Symbol Digit Modalities Test (SDMT).