24 Months Safety and Efficacy Study of AADvac1 in Patients With Mild Alzheimer's Disease

Inclusion criteria (abbreviated): * Diagnosis of probable Alzheimer's disease according to the revised National Institute on Aging-Alzheimer's Association (NIA-AA) criteria (McKhann 2011). * Mini Mental State Examination (MMSE) score ≥ 20 and ≤ 26. * Brain MRI finding consistent with the diagnosis of Alzheimer's disease * Medial temporal lobe atrophy: Scheltens score of ≥ 2 (on a scale of 0-4 on the more atrophied side) AND/OR positive AD biomarker profile in the CSF (amyloid+, tau+) * At least 6 years of formal elementary education. * Age 50-85 years. * Fluency in the local language and sufficient auditory and visual capacities to allow neuropsychological testing. * Ability to read and understand the informed consent. * Stable therapy with an acetylcholinesterase inhibitor for at least 3 months prior to screening. * If the patient is on memantine treatment, the dose regimen must be stable for at least 3 months prior to screening. * Hachinski Ischemia Scale score ≤ 4. * Availability of a caregiver. * Female patients must be either surgically sterile or at least 2 years postmenopausal. * Male patients must either be surgically sterile, or he and his female spouse/partner who is of childbearing potential must be using highly effective contraception starting at screening and continuing throughout the study period. * Patient provides written informed consent. Exclusion criteria (abbreviated): * Participation in another clinical study within 3 months prior to screening. * Pregnant or breastfeeding female. * Not expected to complete the clinical study. * Known allergy to components of the vaccine. * Contraindication for MRI imaging. * Any of the following detected by brain MRI: * Infarction in the territory of large vessels * More than one lacunar infarct. * Any lacunar infarct in a strategically important location. * Confluent hemispheric deep white matter lesions (Fazekas grade 3). * Other focal lesions which may be responsible for the cognitive status of the patient or any other abnormalities associated with significant central nervous disease other than Alzheimer's disease. * Surgery (under general anaesthesia) within 3 months prior to screening and/or scheduled surgery (under general anaesthesia) during the whole study period. * Patient has a history and/or currently suffers from a clinically significant autoimmune disease, or is expected to receive immunosuppressive or immunomodulatory treatment at the present or in the future. * Recent history of cancer (last specific treatment ≤ 5 years prior to Screening). * Myocardial infarction within 2 years prior to screening. * Hepatitis B, C, HIV or Syphilis. * Active infectious disease. * Presence and/or history of immunodeficiency. * Patient currently suffering from a clinically important systemic illness: * poorly controlled congestive heart failure (NYHA ≥ 3) * BMI \> 40 * poorly controlled diabetes (HbA1c \> 7.5%) * severe renal insufficiency (eGFR \< 30 mL/min) * chronic liver disease - ALT (alanine aminotransferase) \> 66 U/L in females or \> 80 U/L in males, AST (aspartate aminotransferase) \> 82 U/L * QTc interval prolongation in ECG (\> 450 ms) * other clinically significant systemic illness, if considered relevant by the investigator * Hypothyroidism, defined as TSH (thyroid-stimulating hormone) elevation \> 5.000 mcIU/mL, and/or FT4 levels \< 0.7 ng/dL. Patients with corrected hypothyroidism are eligible for the study provided that treatment has been stable for 3 months before study entry. * Valid diagnosis of a significant psychiatric illness such as schizophrenia, any type of psychotic disorder or bipolar affective disorder. * Current depressive episode (Geriatric Depression Scale GDS ≥ 6) or major depressive episode within the last 1 years. * Metabolic or toxic encephalopathy or dementia due to a general medical condition. * History of alcohol or drug abuse or dependence within the past 2 years. * Wernicke's encephalopathy. * History or evidence of any CNS disorder other than AD that could be the cause of dementia. * Cerebrovascular disease (ischemic or haemorrhagic stroke), or diagnosis of possible, probable or definite vascular dementia. * Epilepsy. * Treatment with experimental immunotherapeutics including intravenous immunoglobulin within 3 months prior to screening. * Treatment with experimental therapies for AD aiming at disease-modification within 3 months prior to screening. * Patient is currently being treated or was treated in the past with any active vaccines for AD. * Treatment with immunosuppressive drugs. * Change in dose of previous and current medications within the last 30 days prior to Screening (V01), if considered clinically relevant by the investigator. * Vitamin B12 deficiency (serum vitamin B12 \< 191 pg/mL).