Method of producing a temperature sensitive (ts) influenza virus, the method comprising:(a) introducing at least one mutation resulting in an amino acid substitution at a position selected from the group consisting of: PA431 ; PB2630; PA497 ; NP55 ; NP114 ;NP 410; NP 5 °9 ; M1159 and M1183 into an influenza B virus genome; and (b) replicating the mutated influenza virus genome under conditions whereby virus is produced.