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.