An article in Medscape Medical News, June 12, 2014,
Natalizumab Continued Past 24 Months Safely in MS, reports:
Between 2005 and May 2014, among 125,800 patients with MS treated in the postmarketing setting, 462 cases of PML have been reported, of which 23% were fatal, for an incidence of 3.6 cases/1000 treated. The highest prevalence has been in patients receiving natalizumab for 2 to 3 years.
The unofficial chefarztfrau Website lists 464 cases as of May 7.