Inclusion criteria:

Participants must meet all of the following criteria to be included in this study:

1. Male or female, age 55 to 80 years inclusive at the time of informed consent, with a
plasma biomarker result that is predictive of intermediate or elevated brain amyloid
at Screening or known before Screening to have elevated or intermediate amyloid
according to previous PET, cerebrospinal fluid (CSF), or plasma testing

? Those 55 to 64 must have 1 of the following additional risk factors, given the
relatively low rates of amyloid positivity less than (<) 65 years, before screening:

- First degree relative diagnosed with dementia onset before age 75, or

- Known to possess at least 1 apolipoprotein E4 variant (APOE4) allele, or

- Known before screening to have elevated brain amyloid according to previous PET
or CSF testing

2. Global Clinical Dementia Rating (CDR) score of 0 at screening

3. Mini Mental State Examination score greater than or equal to (>=) 27 (with educational
adjustments) at screening.

4. Wechsler Memory Scale-Revised Logical Memory subscale II (WMS-R LM II) score at
screening of >=6

5. A45 Trial: Elevated brain amyloid pathology by amyloid PET: defined as approximately
greater than (>) 40 Centiloids on screening scan A3 Trial: Intermediate levels of
brain amyloid pathology by amyloid PET: defined as approximately 20 to 40 Centiloids
on screening scan

6. Has a study partner that is willing to participate as a source of information and has
approximately weekly contact with the participant (contact can be in-person, via
telephone or electronic communication). The study partner must have sufficient contact
such that the investigator feels the study partner can provide meaningful information
about the participant's daily function

7. Provide written (or electronic, if allowed per country-specific regulations) informed
consent

8. Willing and able to comply with all aspects of the protocol

Exclusion criteria:

Participants who meet any of the following criteria will be excluded from this study:

1. Females who are breastfeeding or pregnant at screening or baseline

2. Females of childbearing potential who:

? Within 28 days before study entry, did not use a highly effective method of
contraception For sites outside of Europe, it is permissible that if a highly
effective method of contraception is not appropriate or acceptable to the participant,
then the participant must agree to use a medically acceptable method of contraception

3. History of transient ischemic attacks (TIA), stroke, or seizures within 12 months of
screening

4. Current or history within the past 2 years of psychiatric diagnosis or symptoms that,
in the opinion of the investigator, could interfere with study procedures

5. Contraindications to 3 Tesla magnetic resonance imaging (MRI) scanning, including
cardiac pacemaker/defibrillator, ferromagnetic metal implants (example, in-skull and
cardiac devices other than those approved as safe for use in MRI scanners), or exhibit
other significant pathological findings on brain MRI at Screening

6. Hypersensitivity to any monoclonal antibody treatment

7. Any immunological disease which is not adequately controlled, or which requires
treatment with immunoglobulins, systemic monoclonal antibodies (or derivatives of
monoclonal antibodies), systemic immunosuppressants, or plasmapheresis during the
study

8. Bleeding disorder that is not under adequate control (including a platelet count
<50,000 or international normalized ratio [INR] >1.5) at screening

9. Results of laboratory tests conducted during screening that are outside the following
limits:

- Thyroid stimulating hormone (TSH) above normal range

- Abnormally low (below lower limit of normal [LLN]) serum vitamin B12 levels for
the testing laboratory (if participant is taking vitamin B12 injections, level
should be at or above the LLN for the testing laboratory). A low vitamin B12 is
exclusionary, unless the required follow-up labs (homocysteine and methylmalonic
acid [MMA]) indicate that it is not physiologically significant

10. Known to be human immunodeficiency virus (HIV) positive

11. Any other clinically significant abnormalities that in the opinion of the investigator
require further investigation or treatment or may interfere with study procedures or
safety

12. Malignant neoplasms within 3 years of screening (except for basal or squamous cell
carcinoma in situ of the skin, or localized prostate cancer in male participants with
treatment cycles completed at least 6 months before screening). Participants who had
malignant neoplasms but who have had at least 3 years of documented uninterrupted
remission before screening need not be excluded

13. Answer "yes" to Columbia-Suicide Severity Rating Scale (C-SSRS) suicidal ideation Type
4 or 5, or any suicidal behavior assessment within 6 months before screening, at
screening, or at baseline, or has been hospitalized or treated for suicidal behavior
in the past 5 years before screening

14. Known or suspected history of drug or alcohol abuse or dependence within 2 years
before screening or a positive urine drug test at screening. Participants who test
positive for benzodiazepines, opioids, or tetrahydrocannabinol (THC) in urine drug
testing need not be excluded unless in the clinical opinion of the investigator this
is due to potential drug abuse

15. Taking prohibited medications

16. Participation in a clinical study involving:

- Any anti-amyloid immunotherapy (example, therapeutic monoclonal antibody or
active anti-amyloid vaccine) at any time, unless it can be documented that the
participant was randomized to placebo or never received study drug

- Any immunoglobulin therapy, or vaccine within 6 months before Screening, unless
it can be documented that the participant was randomized to placebo or never
received study drug

- Lecanemab

- Any new chemical entities or investigational drug for AD within 6 months before
screening unless it can be documented that the participant received only placebo

- Any other investigational medication or device study in the 8 weeks or 5
half-lives (whichever is longer) of the medication before randomization unless it
can be documented that the participant was in a placebo treatment arm

17. Planned surgery during the pre-randomization phase or within 3 months of
randomization, which requires general anesthesia

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

Eligibility last updated 7/11/23. Questions regarding updates should be directed to the study team contact.