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Figure. A) The 8-plasmid reverse genetics system to generate recombinant, live, attenuated pandemic influenza vaccines. Six plasmids encoding the internal genes of the attenuated donor virus are mixed with 2 plasmids encoding the circulating avian virus hemagglutinin (HA) and neuraminidase (NA) genes (which may or may not have been modified to remove virulence motifs). Qualified cells are transfected with the plasmids, and the attenuated reassortant virus is isolated. B). Generation of live, attenuated pandemic influenza vaccine viruses with the 6 internal genes from the attenuated donor virus bearing attenuating mutations (*) and the HA and NA genes from the circulating avian virus by classic reassortment. The 6-2 reassortants generated by this method are selected in the presence of antiserum specific for HA and NA of the attenuated donor virus.