Method for the insertion of protein antigens, of recombinant or synthetic origin, in outer membrane vesicles of Gram-negative bacteria without disruption of the vesicle structure, therefore maintaining the immunogenicity and immunostimulatory properties of said vesicles, and with the reported advantage that the immune response generated against the incorporated antigen is superior to the one generated when the antigen is administered alone. The resultant vaccine formulations are useful to increase protective capacity of existing vaccines and allow to extend it against different pathogens, in diseases of bacterial, viral, cancerous or other etiology. The referred formulations are applicable in the pharmaceutical industry as vaccines for therapeutic and preventive use in humans.