Cold adaptation generates mutations associated with the growth of influenza B vaccine viruses

•HA yields of influenza B viruses used in vaccine manufacture are often unpredictable and inadequate.•Preparation of high-yielding influenza B vaccine seed viruses by reassortment has proven difficult.•Mutations associated with the growth of influenza B vaccine viruses have been identified.•This study has the potential to allow improved predictability in the manufacture of influenza vaccines.