A method for preparing an injectable vaccine composition intended to combat porcine reproductive and respiratory syndrome (PRRS), includes at least the step in which: a) a live vaccine is mixed extemporaneously with an adjuvant diluent (AD). The adjuvant diluent is an oil-in-water-type emulsion or an oil-in-water-type microemulsion, or an aqueous solution including water and at least one inorganic salt selected from aluminum hydroxide, cerium nitrate, zinc sulfate, colloidal iron hydroxide or calcium chloride, salts of divalent or trivalent metals or sympathomimetic compounds.
