As far as efficacy and why I would take a med (Betaseron) that was less effective than Tysabri:
Statistically, Betaseron (and the other interferons) have been shown to be 33 percent effective in reducing exacerbations.
But that 33 percent is a mean. Some people will do better than 33 percent, some people will do worse.
I evidently am one of those that, for now, am responding better than 33 percent to the medication in controlling my exacerbations and progression.
So I know Betaseron is working for me.
Now to the Tysabri, it has an effective rate of 66 percent in reducing exacerbations, but not all people will experience a 66 percent reduction. Some will have a better effectiveness, some worse.
So why would I take the gamble to change meds, when what I am using is working just fine for me. I have no side effects (other than site reactions). I do not have to premedicate (with the interferons many premedicate with ibuprophen to control flu like side effects, with tysabri, many are premedicating with benadryl in case of allergic reaction), and I find it very easy to comply with my chosen treatment.
Efficacy rate is not a guarantee that a particular drug will be better for a given individual who is not experiencing problems with the current therapy, and whose disease is being controlled by the current therapy.