This is because no one has any idea of what would have happened to any particular person. What INF-B is not, is a cure for some people and no good at all for others. The bottom line is that, even if it doesn't appear to be working, it probably is. It is believed that the minority of lesions detectable on MRI scans produce clinically measurable symptoms, but the benefits of these massive reductions may pay dividends years later after onset of a secondary progressive phase. In secondary progressive MS, beta interferon has been shown to significantly reduce the rate of EDSS progression.
Indications are that progression is slowed by around five months per year.
Long term studies show that, for most people, beta interferons continue to be effective with continued use. However a sizeable minority of patients develop neutralising antibodies to the drugs which reduces their effaciacy. Five year studies of beta interferon use have turned up no serious side-effects. This has always struck me as strange. These can be very unpleasant but often subside after a few months and respond well to Ibprofen.