Some people with MS have a dormant Virus
in them called JC Virus.
Tysabri will trigger that Virus, the Virus lays dormant
in the Brain. Until they approve the PRC Blood test for that Virus, there is only a slight way to detect it, and that is with a Brain and CSpine MRI.
Therefore, Tysabri is quite risky. Copaxone, on the other hand is a much "cleaner" treatment, less risky and easier on your system, with about the same amount of side effects as MS itself.
Copaxone is safer.