This is a global multicenter, randomized, double-blind, parallel-group, placebo-controlled study. The treatment period is 96 weeks (2 years) in duration. Treatment Year 1 (Week 0 to Week 48) is referred to as the placebo-controlled treatment period of the study. At the beginning of Treatment Year 1, participants were randomized to receive placebo, peginterferon beta-1a 125 μg every 2 weeks, or peginterferon beta-1a 125 μg every 4 weeks. At the end of Treatment Year 1, participants in the placebo group were re-randomized to receive peginterferon beta-1a treatment so that during treatment Year 2 (Weeks 48 to Week 96) all participants received peginterferon beta-1a 125 μg every 2 or every 4 weeks. Per protocol, all primary and secondary endpoints pertain to Year 1 data.