This clinical trial is focused on studying the effects of a medication called Tenofovir Alafenamide in people with Multiple Sclerosis (MS). MS is a condition that affects the brain and spinal cord, leading to a range of symptoms such as problems with vision, arm or leg movement, sensation, or balance. The study will explore how safe it is to use Tenofovir Alafenamide alongside another medication called Natalizumab, which is already used to treat MS. The trial will also look at the activity of the Epstein-Barr virus (EBV) in people with MS, as EBV is a common virus that can be found in the body.

Participants in the study will receive Tenofovir Alafenamide in the form of film-coated tablets, taken orally, and Natalizumab as a solution for infusion. The study will last for about six months, during which time the safety of the treatment combination will be closely monitored. The trial aims to understand if adding Tenofovir Alafenamide to the current treatment with Natalizumab is safe and how it affects the presence of EBV in the body.

Throughout the study, researchers will observe any serious side effects and changes in the levels of EBV in saliva. The study will also measure changes in certain antibodies, which are proteins made by the immune system to fight infections. These observations will help determine the impact of the treatment on the virus and the overall health of the participants. The goal is to gather information that could lead to improved treatment options for people with MS.