OUR PURPOSE PIONEERING SCIENCE PATIENTS OUR PEOPLE ENVIRONMENT COMMUNITY REPORTING 7 CEO LETTER BIOGEN 2021 YEAR IN REVIEW This is not the first time a new class of therapy on the cutting edge of scientific discovery has faced headwinds. We continue to believe that ADUHELM represents the first major innovation in decades and the beginning of what we hope will be a wave of new developments in this area. To this end, we continue to advance lecanemab, another amyloid-beta targeting investigational therapy under development in our collaboration with Eisai Co., Ltd. (Eisai). In June 2021, the FDA granted Breakthrough Therapy designation for lecanemab, and in December 2021 lecanemab also received Fast Track designation. The readout of the Phase 3 confirmatory Clarity AD clinical trial is expected in the fall of 2022, and it is currently under rolling submission for a Biologics License Application (BLA) under the accelerated approval pathway, with filing expected to be completed in the second quarter of 2022. When additional data from this new class of treatments become available, we will request that CMS reconsider its coverage decision for all FDA-approved amyloid-beta targeting therapies. Biogen continues to advocate for patients to have rapid and equitable access to all FDA- approved therapies to treat Alzheimer’s disease and for the continuity of care for Medicare beneficiaries already being treated with ADUHELM. Beyond our programs targeting amyloid, we are pursuing a multi- modality approach focused on other targets in Alzheimer’s disease. In a Phase 1b study, BIIB080, a tau-directed ASO, met the primary objective of safety and tolerability. Furthermore, data demonstrated a durable time and dose-dependent reduction of tau protein in cerebrospinal fluid and, based upon these results, we expect to initiate a Phase 2 study. Growing evidence suggests tau may be a key driver of neurodegeneration in Alzheimer’s disease. In addition, in February 2022 we initiated a Phase 1 study for BIIB113, a small molecule with a mechanism of action that targets tau protein aggregation. 2021 Pipeline Highlights We received significant data readouts during the year in a number of promising areas. We see high potential for zuranolone (BIIB125), an investigational two-week, once-daily drug in development for the potential treatment of major depressive disorder (MDD) and postpartum depression (PPD), to help transform the treatment of these diseases. Zuranolone now has positive data from five randomized clinical studies in MDD and PPD. Given the strength of this clinical data and the potentially differentiated profile of zuranolone, we are preparing to start, together with Sage Therapeutics, Inc. (Sage), a rolling submission with the FDA for a New Drug Application for the potential treatment of MDD. We expect to complete the filing in the U.S. in the second half of this year. In stroke, we received positive results from the Phase 2 study of BIIB131 (TMS-007), in acute ischemic stroke. Acute ischemic stroke accounts for about 87% 1 of all strokes worldwide. Unfortunately, approved thrombolytic agents, the current standard of care, are limited in their use due to their benefit-risk profile in later time windows where they are administered within 3 to 4.5 hours 1 of symptom onset. During the Phase 2a study, patients were dosed 4.5 to 12 hours after the onset of stroke symptoms, with treatment times averaging approximately 9.5 hours. Patients who received BIIB131 experienced no symptomatic intracranial hemorrhage. In addition, the study demonstrated significant impacts on blood vessel reopening and patient functional recovery. As we advance BIIB131, we also have BIIB093 (glibenclamide IV), currently in Phase 3, being studied for the treatment and prevention of severe cerebral edema in large hemispheric infarction, one of the most severe types of ischemic stroke 2 where cerebral edema often leads to high morbidity and mortality. In lupus, we announced the first patient dosed in the Phase 3 study of BIIB059 in systemic lupus erythematosus (SLE). SLE is a chronic autoimmune disease that affects multiple organ systems. The Phase 3 study will evaluate the clinical efficacy and assess the safety of BIIB059 as compared to placebo. We have set enrollment targets for the study that reflect the high prevalence of SLE in Black or African American and Hispanic and Latino communities, which are disproportionately impacted by the disease. ALS also remains an area of focus for Biogen, where we continue to engage with regulators to evaluate the next step for tofersen in SOD1 ALS. In the Phase 3 study, the primary endpoint as measured by the Revised Amyotrophic Lateral Sclerosis Functional Rating