• Male or female participants aged 55 to 80 years (inclusive) at Screening or aged 30 to 80 years (inclusive) if presence of a historically documented dominantly inherited mutation associated with monogenic AD. - Clinical diagnosis of mild cognitive impairment (MCI) due to AD or mild AD with MMSE 18-30 and CDR Global score of 0.5-1. - Evidence of amyloid and tau pathology consistent with AD diagnosis by both: - Apart from the clinical diagnosis of early AD, participant must be in good health as determined by the Investigator. - If the participant is receiving an approved symptomatic AD treatment, such as acetylcholinesterase or NMDA inhibitors, the participant must be on a stable dose for at least 8 weeks prior to Screening and until Day 1. - Stable doses of all other (non-AD-related) concomitant medications for at least 4 weeks prior to Screening and until Day 1. - Must have an identified reliable Study Partner.

Any medical or neurological/neurodegenerative or psychiatric condition (other than AD) that may be a contributing cause to cognitive impairment or could confound interpretation of drug effect, affect study assessments, or affect participant's ability to participate and complete the study or lead to safety concerns. - Seropositive for anti-AAV9 antibodies at Screening. - History of transient ischemic attack or stroke or any unexplained loss of consciousness within 1 year prior to Screening. - History of seizures within 10 years prior to Screening or history of epileptic syndrome (except for history of febrile seizures in childhood). - Presence of a clinically significant uncontrolled medical disorder that may compromise the participant's safety or their ability to complete all of the study assessments. - History of significant cardiovascular disease. - Contraindications to lumbar puncture, MRI imaging, PET imaging or corticosteroids. - History of, or positive test result for human immunodeficiency virus (HIV), hepatitis C or current acute hepatitis B. - History within 1 year prior to screening of drug or alcohol abuse. - History of severe allergies, or history of an anaphylactic reaction (nonactive hay fever is acceptable). - Previous or current use of an approved AD disease-modifying therapies - Previous or current participation in a clinical study involving any cell or gene therapies (including but not limited to AAV-based gene therapies) or active immunotherapies targeting Tau or amyloid, or any anti-amyloid or anti-Tau therapies or any therapeutic mAb, protein derived from a mAb, immunoglobulin therapy, antisense oligonucleotides, small interfering ribonucleic acid, or any other agent with purported disease-modifying effect in AD unless it can be documented that the participant only received placebo..