I don't know that much about tysabri. It is a progression therapy just like copaxone, rebif, betaseron and avonex. Instead of taking injections, you receive it in an IV infusion once a month. It is not an interferon.
Anecdotal records seem to indicate that it is particulary good at reducing fatigue. It is what many docs go with when another med is not being successful.
I am sure others will be along with more info. That is all I know.
Love and prayers,