I think the number is as high as 40%.
Rebif is the next most likely with something like 23%.
Avonex is the least likely with an incidence of NA's of less than 10%. The problem appears to be dose-related. Betaseron is the highest dose, Rebif next and Avonex the lowest.
Once you have neutralizing antibodies to one of the Interferon-beta meds, you have it to ALL of them.
If JohM is around, he has doen the most research into this problem, if I remember right.