via CCSVI in Multiple Sclerosis -
"NEW: Updated PML risk numbers--1 per 330--
With more than 115,000 patients globally treated with natalizumab (Tysabri) for longer periods of time, that estimate of PML risk is 1 per 330. In patients who test positive for antibodies to JCV, have a clinical history of immune suppressive treatment before natalizumab, and have received more than 24 doses, the number of PML cases is 1 per 90." http://www.ncbi.nlm.nih.gov/pubmed/23925759
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