Influenza A viruses are important pathogens of humans and animals. Despite the availability of inactivated virus vaccines, extensive mortality and morbidity are caused by this virus in humans. Year-to-year antigenic variation of the circulating human influenza virus strains makes it necessary to change the vaccine formulation frequently. This is specifically important when an antigenic shift occurs in which a novel influenza A virus emerges containing antigenic determinants derived from circulating avian strains. In this case, an influenza epidemic could emerge that would cause extensive morbidity and mortality unless a new vaccine could be prepared in protect the susceptible population. The classical process of developing a new viral vaccine strain usually requires the generation of a reassortant influenza virus containing six genes derived from a high-yield virus strain and two genes (hemagglutinin and neuraminidase) encoding the major protective antigenic determinants from the new strain. We have now developed a plasmid-based technology that will significantly shorten the time required to generate new vaccine virus candidates. We propose to use this technology to generate candidate vaccines based on live attenuated virus strains and/or high-yield viruses in tissue culture systems. The attenuation strategy for development of attenuated influenza viruses by targeted gene modification will be employed. At the completion of this project it is expected that four new vaccines will be developed and clinical batches prepared in cell culture ready for clinical studies. These vaccines will contain antigenic determinants derived from a model avian influenza pandemic strain. The proposed R&D effort will result in development of novel and improved strategies for genetically engineering new influenza vaccines and specifically vaccines of specific serotype in event of a pandemic caused by a new avian influenza virus strain.