In the context of multiple sclerosis (MS), where PML is often linked to immunosuppressive therapies like natalizumab, survival rates are generally better: around 75-92% overall, with even higher rates (up to 97%) if detected asymptomatically through routine monitoring.      One population-based study reported a one-year mortality rate of about 38% across all PML cases, including MS.  However, survivors frequently experience lasting neurological disabilities, such as cognitive impairment or motor deficits.  

"According to available medical literature and pharmaceutical reporting, there is no single definitive global registry tracking every case of progressive multifocal leukoencephalopathy (PML) in multiple sclerosis (MS) patients, as PML is a rare complication often linked to specific disease-modifying therapies (DMTs). However, cumulative confirmed cases can be estimated from drug-specific reports, as PML in MS is predominantly associated with immunosuppressive DMTs rather than MS itself. Based on the most recent data:
• Natalizumab (Tysabri): Approximately 857 confirmed PML cases worldwide, with 233 in the US and 624 in the rest of the world.  This accounts for the vast majority of reported cases in MS patients.
• Fingolimod (Gilenya) and related S1P modulators: Around 50-60 confirmed cases. 
• Dimethyl fumarate (Tecfidera): Approximately 12 confirmed cases. 
• Ocrelizumab (Ocrevus): 18 confirmed cases as of May 2025, with most attributed to carry-over effects from prior DMTs. 
• Rituximab (used off-label for MS): Around 10 reported cases. 
• Other DMTs (e.g., alemtuzumab): 1-5 cases total. "