The NS1 protein of influenza virus is a key virulent element with multi-functional roles in virus replication and acts as a strong interferon (IFN) antagonist. A live attenuated virus (LAIV) is provided using a master backbone, which contains the influenza B (HK8038) virus and includes a deletion of the NSI coding region (DelNS1). The LAIV is based on novel adaptive mutations, which support DelNS1 influenza B live attenuated virus (LAIV) replication in vaccine producing cells. DelNS1 influenza B LAIV shows spontaneous cold adaption with preference to grow at 30-33° C. but restriction at 37-39° C. The LAIV can be used to protect a subject, against a lethal challenge of antigenic distant influenza B viruses. DelNS1 LAIV with adaptive mutations for growing in vaccine producing systems is an important strategy for making highly attenuated and immunogenic live attenuated influenza vaccines with the ability to induce broad cross protective immunity for seasonal influenza.

The invention relates to a method for producing influenza infectious viruses wherein CHO cells are infected with a seed of infectious influenza virus which has been generated by transfecting cells with an appropriate set of expression vectors. The invention also relates to a recombination cassette, and to a vector comprising said recombination cassette, that may be used in methods for producing infectious viruses, and particularly in the method according to the invention.