It’s been three decades since scientists hypothesized that Alzheimer’s is caused by a buildup of amyloid beta protein in the brain. But it wasn’t until last month that drugmakers achieved a major breakthrough, with Japan’s Eisai Co. and its US partner Biogen Inc. releasing results from a large-scale trial showing they were able to blunt the disease’s progression. The companies are seeking accelerated US approval for the medication, lecanemab, though questions remain about the extent of the benefits, side effects, and how it would be covered by insurers if approved.