If you choose to go off of Tysabri, there is going to be another washout period. The recommended length for
Tysabri to
gilenya, is about 4 months. During that time, the recommended protocol is to trreat the patient with routine rounds of steroids until the gilenya can be started. I asked this question recently and the washout to go to the other DMDs can be about 30 days, just not the jump to Gilenya.
The Incidence of PML is small, but that is no comfort to those people who contraact it.