Study to Evaluate the Safety and Tolerability of a New Drug Named Lu AF20513 in Patients With Mild Alzheimer's Disease

Main Inclusion Criteria: * The patient has a diagnosis of probable Alzheimer's Disease (AD) consistent with the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) criteria * The patient has a pattern of antibodies in cerebrospinal fluid (CSF) consistent with an amyloid plaque load * The patient is a man or a woman, and between ≥60 and ≤85 years of age * The patient has an MRI (performed within 3 months before screening) with results consistent with the diagnosis of probable AD * The patient has a mild severity of dementia * The patient has a knowledgeable and reliable caregiver who will be available and able to: accompany the patient to all clinical visits, monitor IRE after each immunization, and participate with the patient at all phone visits during the study AD * Patients must have completed Part A before being eligible for continued immunisations in Part B Main Exclusion Criteria: * The patient has evidence of mixed etiology of dementia (i.e. absence of other neurodegenerative, neuroinflammatory or cerebrovascular disease, or another neurological, mental or systemic disease or condition likely contributing to cognitive decline) * The patient has clinical and radiological findings that fulfil the standards of the National Institute of Neurological Disorders and Stroke and Association Internationale pour la Recherché et l'Enseignement en Neurosciences (NINDS-AIREN) criteria for vascular dementia * The patient has a Diagnostic and Statistical Manual of Mental Disorders, 4th edition, Text Revision (DSM-5-TR) Axis I disorder other than AD; including amnestic disorders, delirium, schizophrenia or schizoaffective disorder, bipolar disorder, current major depressive episode, psychosis, panic, post-traumatic stress disorder and/or cognitive disorder not otherwise specified, (note: patients may be included if treated with a stable dose of antidepressants for at least 2 months and not fulfilling DSM-5-TR criteria for depression at Screening) * The patient's eligibility MRI scan (1.5T) shows findings that correspond to more than 4 brain micro haemorrhages * The patient has extensive white matter lesions as shown on the screening MRI scan (1.5T) Other protocol-defined inclusion and exclusion criteria do apply