A method for obtaining a vaccine against the infection of an animal host by a retrovirus capable of penetrating into a host's target cell, where the target cell possesses a membrane receptor for a protein of the host, by preparing candidate vaccine agents based on a modified polypeptide having at least part of an envelope protein of the retrovirus, where the polypeptide has at least one fragment of an immunodominant region of the envelope protein, the fragment containing at least one conserved amino acid of the immunodominant region present in the pathogenic strain, wherein the polypeptide induces an immune response directed both against the immunodominant region and against the protein of the host; and selecting as a vaccine such a modified polypeptide chosen from those which induce an immune response directed against said immunodominant region of the envelope protein and not against the protein of the host.