Medicare plans to cover Biogen drug only for patients in clinical trials

Medicare will only cover the cost of Aduhelm, the Alzheimer's drug made by Biogen Inc., for certain patients enrolled in clinical trials if the feds stick to a preliminary decision issued by the Centers for Medicare and Medicaid Services (CMS). The federal agency posted a proposed decision to its website on Tuesday afternoon. Biogen (Nasdaq: BIIB) shares began to slide in after-hours trading, dropping by about 6% in the first hour after the closing bell. The final CMS decision will not come until…