WEBAt 54 sites across North America, Japan, Australia, and Europe, participants will receive high- or low-dose BIIB080 every 24 weeks, high dose every 12 weeks, or placebo, for 72 weeks. The primary outcome is the dose response in change of CDR-SB from baseline after 76 weeks. Secondary measures include ADCS-ADL-MCI, ADAS-Cog 13, MMSE, iADRS ...