The invention provides a method of searching for and obtaining a vaccine against the pathogenic effects related to the infection of an animal or human host by a retrovirus that penetrates into a target cell of the host, and a vaccine obtained by the method are provided. The method includes preparing candidate vaccine agents based on a polypeptide comprising at least part of an envelope protein of a pathogenic strain of the retrovirus and selecting as the vaccine a modified polypeptide chosen from polypeptides that induces an immune response directed against an immunodominant region of an envelope protein of the retrovirus and not against a protein of the host.