This study is testing a new vaccine called AV-1959D to see if it's safe for people with early Alzheimer's disease (AD). A vaccine is a type of medicine that helps your body fight illnesses. The study is a "Phase 1 trial," which means it's the first time this vaccine is being tested in humans. Participants will be randomly assigned to receive either the vaccine or a placebo (a harmless substance with no medical effects) to compare results. The trial will be conducted across multiple centers and will include 3 different groups to test varying doses.

• Participation requires 60 to 85-year-olds with early signs of Alzheimer's disease.
• Participants will need to attend clinic visits with a study partner who knows them well.
• Those involved must not have participated in another drug study recently.

Before joining, participants must have a brain scan called a PET scan to check for certain signs of Alzheimer's. There are also specific health conditions that might prevent someone from joining, like certain heart problems or recent serious illnesses. Participants and their study partners need to give their consent to be part of the study.