All i can surmise is possibly
patients and doctors are not choosing
Tysabri when they their is a high risk factor.....
"For JCV antibody–positive patients with no prior immunosuppressant use, the risk for PML increased with longer natalizumab use. For 1 to 24 months of exposure to the drug, the risk was 0.6/1000. With 25 to 48 months of exposure, the risk increased to 5.2/1000, and with 49 to 72 months of natalizumab, the risk was 5.4/1000.