Abstract: A recombinant influenza virus strain derived from (a) an over-attenuated influenza A strain which has been passaged solely in chicken egg cultures, manifests a relatively high yield of virus in such cultures and has an increased capacity for plaque formation on calf kidney cells and (b) a virulent influenza A strain which manifests a relatively low culture yield of virus and is substantially free from contaminating mammalian viruses; a method of manufacturing the recombinant strain and a vaccine incorporating it.The virulent strain is preferably one fairly recently isolated at the time of manufacture of the vaccine. The over-attenuated strain should have sufficient antigenic distinction between it and the virulent strain and have a proven safety record.