Disclosed are interactions between different betanodaviruses during infection of cells “in vitro” and in European sea bass. More specifically, fish administered striped jack nervous necrosis viruses (SJNNV), which are then exposed to red-spotted grouper nervous necrosis viruses (RGNNV), have fewer symptoms of disease associated with RGNNV and/or increased survival as compared to fish not administered SJNNV prior to exposure to RGNNV.