A Study to Evaluate Safety and Tolerability of Aducanumab in Participants With Alzheimer's Disease Who Had Previously Participated in the Aducanumab Studies 221AD103, 221AD301, 221AD302 and 221AD205

Key Inclusion Criteria:

Core Treatment Period:

1. Participant was participating in an aducanumab clinical study at the time of the announcement of early termination (feeder studies).
2. Has one care partner who, in the Investigator's opinion, has adequate contact with the participant as to be able to provide accurate information about the participant's cognitive and functional abilities.

LTE Treatment Period:

1. Participant must have completed the Core study period (Week 102) and adequately tolerated 10 mg/kg of aducanumab during the Core study period in the opinion of the Investigator.
2. Has one informant/care partner who, in the Investigator's opinion, has frequent and sufficient contact with the participant as to be able to provide accurate information about the participant's cognitive and functional abilities.

Key Exclusion Criteria:

Core Treatment Period:

1. Any medical or neurological condition (other than Alzheimer's Disease) that might be a contributing cause of the participant's cognitive impairment.
2. Stroke or any unexplained loss of consciousness within 1 year prior to Screening.
3. Clinically significant unstable psychiatric illness in past 6 months.
4. History of unstable angina, myocardial infarction, advanced chronic heart failure, or clinically significant conduction abnormalities within 1 year prior to Screening.
5. A seizure event that occurred after the last visit of the feeder study and before Screening for this study.
6. Evidence of impaired liver function as shown by an abnormal liver function profile at Screening.
7. History of or known seropositivity for HIV.
8. Clinically significant systemic illness or serious infection within 30 days prior to or during Screening.
9. Contraindications to having a brain magnetic resonance imaging (MRI).

LTE Treatment Period:

Any medical or psychiatric contraindication or clinically significant abnormality that, in the opinion of the Investigator, will substantially increase the risk associated with the participant's enrollment in and completion of the study.

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