A vaccine for delaying onset of or for treatment of Alzheimers disease or an Alzheimer-related disorder in an individual comprises a therapeutically effective amount of a physiologically acceptable protofibril/oligomer comprising N-terminal truncated Aβ. An antibody for delaying an onset of or for treatment of Alzheimers disease or an Alzheimer-related disorder in an individual binds one or more truncated AO protofibrils/oligomers, but exhibits no or substantially no cross-reactivity with full length Aβ monomers, and optionally said antibody shows cross-reactivity to N-terminal truncated Aβ monomers. Methods for delaying an onset of or for treatment of Alzheimers disease or an Alzheimer-related disorder employ the vaccine or antibody. Methods of detecting soluble N-terminal truncated amyloid-beta (Aβ) protofibrils/oligomers and N-terminal truncated Aβ monomers employ the antibody.
