Longevity of Tysabri benefits

I have been on Tysabri for four years now. I know some people have had seemingly miraculous results after their infusions. I for one have taken Tysabri to slow the progression of this darn disease and have never felt any different before, during or after my infusion. But my disease activity has been minimal, so that is success to me. Biogen has come out with a blood test trial telling you if you have antibodies or the JC virus in your system. If that comes back negative, I say continue with the Tysabri as it seems to be the best product on the market, in my opinion.