IS = previous exposure to immunosuppressive drugs
"There have now been over 210,000 patient years of exposure. As of the 6th June 2012 there are 258 confirmed cases of natalizumab-associated cases of PML. The risks are beginning clearer with time, as noted by the decreasing size of the error bars in the figure above. The smaller the error bars the more certain the estimate."
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CoI: multiple, these data have been provided by Biogen-Idec