biogen inc tumbled on friday as setbacks for its alzheimer’s disease drug aduhelm helped erase all the gains the stock had made in 2021 the biotech stock has plunged more than 40 after reaching a sixyear high in june when aduhelm was approved the decline has wiped out more than 26 billion in biogen’s market value on friday it sank 29 amid a broader market selloff over concerns surrounding a new covid variant