Vectors and methods for the production of influenza viruses suitable as recombinant influenza vaccines in cell culture are provided. Bi-directional expression vectors for use in a multi-plasmid influenza virus expression system are provided. Additionally, the invention provides methods of producing influenza viruses with enhanced ability to replicate in embryonated chicken eggs and/or cells (e.g., Vero and/or MDCK) and further provides influenza viruses with enhanced replication characteristics.
