Production of viruses viral isolates and vaccines

A method for producing an influenza virus particle, said method comprising the steps of:

• contacting a human embryonal retina cell transformed with E1 of an adenovirus with an influenza virus particle in a culture medium under conditions conducive to infection of said cell by said influenza virus particle, wherein the influenza virus particle is from an influenza isolate obtained from a subject infected with influenza virus;
• and
• culturing said cell under conditions conducive to propagation of said influenza virus particle, wherein said cell comprises a heterologous nucleic acid encoding an alpha2,6 sialyltransferase or a functional equivalent thereof and overexpresses the alpha2,6 sialyltransferase or functional equivalent thereof as compared to the cell that does not comprise the heterologous nucleic acid.