Researchers want to know if the study treatment called MK-2214 works to slow certain changes in the brains of people with Alzheimer's disease (AD). AD is a type of dementia that can cause loss of memory, communication (such as speech), and decision-making skills. It can limit a person's ability to do daily tasks. MK-2214 is a study treatment designed to slow down AD. The goals of the study are to learn: * If MK-2214 slows the spread of tau in the brain compared to placebo. Tau is a protein that accumulates in AD \& damages brain cells. A placebo looks like the study treatment but has no study treatment in it. Using a placebo helps researchers better understand the effects of a study treatment. * About the safety of MK-2214 and if people tolerate it