The purpose of this study is to better understand how certain parts of the immune system may explain differences seen in the severity of Multiple Sclerosis. This will require one visit, typically less than an hour. We will collect some information about the subject's past medical history including the severity of their multiple sclerosis, and their medications as well as collect a sample of blood. This will also include a measure of genetics related to the immune system. Blood will also be stored for future research.

The main purpose of the study is to find out if patients treated with remibrutinib may experience fewer Multiple Sclerosis relapses (also called clinical attacks, exacerbations or flare ups) than patients treated with teriflunomide (also known as Aubagio). Teriflunomide is an approved medication for the treatment of relapsing MS. This clinical study will have two parts. The first part is the masked part in which neither you nor your doctor know if you receive remibrutinib or teriflunomide and will last up for a maximum of up to 30 months (~2.5 years). This is followed by an Extension Part which will have treatment and safety follow up periods; during the treatment period remibrutinib treatment is provided to all participants who had completed core part of study for a maximum of up to 5 years.