ZUG, Switzerland & DUBLIN, Ireland--(BUSINESS WIRE)--Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN) announced today that data from the Phase III AFFIRM monotherapy study demonstrated that treatment with TYSABRI® (natalizumab) significantly reduced the proportion of multiple sclerosis (MS) patients with worsening cognitive function as measured by the 3-second Paced Auditory Serial Addition Test (PASAT 3). These data, presented at the 22nd Congress of the European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS) in Madrid, Spain, contribute to existing data which demonstrate the overall therapeutic benefits of TYSABRI, including its significant impact on relapse reduction, disability progression and MRI measures. TYSABRI has demonstrated a 68% relative reduction in the annualized relapse rate compared to placebo and a 42% reduction in the relative risk of disability progression, as published in the New England Journal of Medicine.