what I've read is that some people develop neutralizing antibodies to the interferon, which is thought to diminish the drug's efficacy. These can be tested for in the blood.
I've been on rebif for 4.5 years. My only relapses were minor and occurred during the first year. I've been 100% symptom-free for quite some time now. I've chosen to believe that the drug is working for me and so I endure the pain of the shots. thankfully, no real other side effects from the drug for me.
good luck with your research.