What percentage of MS patients are on tysabri (natalizumab)? Is there a big chance of bad side effects?

Answer continues... If, you have tested negative for jc virus antibodies, your chance of getting pml is around 0.09/1, 000. On the other hand, if you have tested positive for jc virus antibodies, have been on tysabri (natalizumab) therapy for over two years and have prior exposure to immunosuppressant therapies, such as azathioprine, methotrexate, micophenolate, Cyclophosphamide or mitoxantrone, your risk becomes 11.1/1000.
See answer below. The percentage of patients on tysabri (natalizumab) vary from clinic to clinic. Tysabri (natalizumab) is usually reserved for use as a second line therapy. It is however the best available treatment for ms patients today (6/23/12). The main side effect you will hear about is pml. The risk for development of pml depends on duration of tysabri (natalizumab) treatment, prior exposure to immunosuppressant therapies and jc virus status.
See below. Not clear to me as to percentage of the various agents, but figures from Biogen suggest over 100,000 worldwide patents and at least 450 cases of PML. This can be managed readily by a risk factor blood test, called the anti-JCV antibody index. Risk of PML runs from 1in 90 to 1 in 40,000. Risk increases with prior use of immunosuppressant agent, Tysabri (natalizumab) >2yrs, positive blood test.