Increase in viral yield in eggs and MDCK cells of reassortant H5N1 vaccine candidate viruses caused by insertion of 38 amino acids into the NA stalk

► We generate H5N1 vaccine viruses with 20aa- or 38aa-insertion into NA stalks. ► We compare the growth properties of these vaccine viruses in eggs and MDCK cells. ► 38aa-insertion into NA stalk can significantly increase viral yield. ► 20aa-insertion into NA stalk is not sufficient to increase viral yield. ► 38aa-insertion into NA stalk is a more economical strategy for vaccine production.