Compared to interferon beta-1a, alemtuzumab reduced the relapse rate by 49%-74% (P < 0.0001), and in two studies it reduced the risk of sustained disability accumulation by 42%-71% (P < 0.01). In one study (Comparison of Alemtuzumab and Rebif Efficacy in Multiple Sclerosis; CARE-MS1), there was no significant difference compared to interferon, perhaps reflecting the surprisingly low frequency of disability events in the comparator group.