To the Editor: Dr Shirani and colleagues1 suggested that interferon beta treatment was not associated with a reduction in progression of long-term disability in patients with multiple sclerosis (MS). In contrast, long-term follow-up of a randomized controlled trial of interferon beta-1b reported a 60% to 70% reduction in long-term disability2 and a 46% to 47% reduction in mortality.3 Although Shirani et al1 did not cite these studies, both reports used bias-reduction methods. Time zero was the same for all patients (either eligibility for interferon beta treatment or trial entry) so that there was no immortal time bias,4 and both analyses used propensity score (covariate) adjustment.5
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