A relapse is defined as new or recurrent neurologic symptoms not associated with fever or infection, lasting for at least 24 hours, and accompanied by new objective neurologic findings. Only relapses confirmed by an independent neurology evaluation committee (INEC) are included in the analysis. Data after participants switched to alternative multiple sclerosis (MS) medications are excluded. Data were analyzed using negative binomial regression, adjusted for baseline Expanded Disability Status Scale (EDSS) score (< 4 versus ≥ 4), baseline age (< 40 versus ≥ 40 years), and baseline relapse rate (number of relapses in 3 years prior to study entry divided by 3).

Intent-to-treat (ITT) population: participants who were randomized and received at least 1 dose of study treatment (peginterferon beta-1a or placebo).

Number of new or newly enlarging T2 hyperintense lesions on brain magnetic resonance imaging (MRI) scans. Data observed after participants switched to alternative MS medications are excluded. Adjusted mean is based on negative binomial regression, adjusted for baseline number of T2 lesions.

ITT population, with at least 1 post-baseline assessment. Missing data prior to alternative MS medications and visits after participants switched to alternative MS medications imputed based on previous visit data assuming the constant rate of lesion development or group mean at same visit.

A relapse is defined as new or recurrent neurologic symptoms not associated with fever or infection, lasting for at least 24 hours, and accompanied by new objective neurologic findings. Only relapses confirmed by INEC were included in the analysis. Estimated proportion of participants relapsed is based on the Kaplan-Meier product limit method.

ITT population: participants who were randomized and received at least 1 dose of study treatment (peginterferon beta-1a or placebo). Participants who did not experience a relapse prior to switching to alternative MS medications or withdrew from study were censored at the time of switch/withdrawal.

Sustained disability progression is defined as: at least a 1.0 point increase on the EDSS from baseline EDSS ≥ 1.0 that is sustained for 12 weeks, or at least a 1.5 point increase on the EDSS from baseline EDSS = 0 that is sustained for 12 weeks. The EDSS measures the disability status of people with MS on a scale that ranges from 0 to 10. The range of main categories include 0 (normal neurologic examination), to 5 (ambulatory without aid or rest for 200 meters/disability severe enough to impair full daily activities), to 10 (death due to MS). Estimated proportion of participants with progression based on the Kaplan-Meier product limit method.

ITT population: participants who were randomized and received at least 1 dose of study treatment (peginterferon beta-1a or placebo). Participants were censored at the time of withdrawal/switch if they withdrew from study or switched to alternative MS medication without a progression.