"Cumulatively, in the post-marketing setting:


Approximately 52,000 patients have been treated with TYSABRI; and,
Of those patients, approximately 24,900 have received at least one year of TYSABRI therapy, approximately 14,400 patients have received at least 18 months of TYSABRI therapy, and 6,800 patients have received at least 24 months of TYSABRI therapy. "

From http://finance.yahoo.com/news/Biogen...-14949365.html

There have been 5 reported PML cases, with 4 occurring between 12 and 17 months of use and 1 occurring at 26 months of use. 4 of the PML cases occurred in Europe which does not have a formal TOUCH program.