A Study of Aducanumab in Participants With Mild Cognitive Impairment Due to Alzheimer's Disease or With Mild Alzheimer's Disease Dementia to Evaluate the Safety of Continued Dosing in Participants With Asymptomatic Amyloid-Related Imaging Abnormalities

Inclusion/ Exclusion Criteria Key Inclusion Criteria: * Ability of the participant or his/her legally authorized representative to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use confidential health information in accordance with national and local participant privacy regulations. * Must have at least 6 years of education or work experience to exclude mental deficits other than MCI due to AD or mild AD dementia. * Must have evidence of cerebral Aβ accumulation, based on a positive PET scan of the brain. Previously obtained positron emission tomography (PET) scan (within 12 months of screening) is permissible. Previous PET scan images must be submitted to the central imaging vendor to confirm that study inclusion criteria are met. * Must consent to apolipoprotein E (ApoE) genotyping. * Must meet all of the following clinical criteria for MCI due to AD or mild AD dementia according to NIA-AA criteria \[Albert 2011; McKhann 2011\], and must have the following: MCI due to AD (a CDR global score of 0.5, and an MMSE score between 24 and 30 (inclusive)), or Mild AD dementia (a CDR global score of 0.5 or 1, and as MMSE score between 20 and 26 (inclusive)). Key Exclusion Criteria: * Any uncontrolled medical or neurological/neurodegenerative condition (other than AD) that, in the opinion of the Investigator, might be a contributing cause of the participant's cognitive impairment (e.g., substance abuse, vitamin B12 deficiency, abnormal thyroid function, stroke or other cerebrovascular condition, Lewy body dementia, frontotemporal dementia, head trauma). * Clinically significant unstable psychiatric illness (e.g., uncontrolled major depression, uncontrolled schizophrenia, uncontrolled bipolar affective disorder) within 6 months prior to Screening. * Transient ischemic attack or stroke or any unexplained loss of consciousness within 1 year prior to Screening. * Vaccinations within 10 days prior to randomization (Day 1). * Female participants who are pregnant or currently breastfeeding. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply