The present invention is based on the discovery that a high titer
reassortant influenza virus is produced in mammalian cell culture by
replacing the NS gene of the A/PuertoRico/3/24 master strain with the NS
gene of the A/England/1/53 strain. The invention provides influenza
viruses and vaccines generated in mammalian cells as well as methods for
producing such. The invention further provides an influenza virus master
strain and kits for generating reassortant influenza viruses in mammalian
cell culture and methods of making and using the master strain.