Alzheimer Disease/drug therapy; Amyloid beta-Peptides/antagonists & inhibitors; Brain/drug effects; Immunotherapy, Active/adverse effects; Meningoencephalitis/chemically induced; Peptide Fragments/antagonists & inhibitors; Vaccines/adverse effects; Aged; Aged, 80 and over; Alzheimer Disease/immunology; Alzheimer Disease/physiopathology; Amyloid beta-Peptides/immunology; Amyloid beta-Peptides/metabolism; Antibodies/blood; Antibodies/immunology; Brain/immunology; Brain/pathology; Cognition Disorders/drug therapy; Cognition Disorders/immunology; Cognition Disorders/physiopathology; Double-Blind Method; Drug Administration Schedule; Female; Humans; Immunotherapy, Active/methods; Injections, Intramuscular; Magnetic Resonance Imaging; Male; Meningoencephalitis/immunology; Meningoencephalitis/physiopathology; Middle Aged; Neuropsychological Tests; Peptide Fragments/immunology; Peptide Fragments/metabolism; Placebos; Treatment Outcome; Vaccines/administration & dosage; Vaccines/immunology