Multi plasmid system for the production of influenza virus

1. A helper virus free method for producing infectious influenza B viruses in cell culture, the method comprising:

i) electroporating a population of Vero cells with a plurality of plasmids comprising nucleic acid sequences corresponding to an influenza B virus genome;

ii) culturing the population of Vero cells under conditions permissive for viral replication in the absence of helper virus; and,

iii) recovering a plurality of infectious influenza B viruses.

2. The method of claim 1 , wherein the influenza B viruses possess one or more phenotypic attributes selected from the group consisting of: attenuation, temperature sensitivity and cold adaptation.

3. The method of claim 1 , wherein the influenza B viruses comprise an attenuated, cold adapted, temperature sensitive influenza virus.

4. The method of claim 1 , wherein the influenza B virus is suitable for administration in an intranasal vaccine formulation.

5. The method of claim 1 , comprising culturing the population of host cells at a temperature less than or equal to 35° C.

6. The method of claim 1 , comprising culturing the Vero cells in serum free medium.

7. The method of claim 1 wherein the plurality of plasmids is eight plasmids.

8. A helper virus free method for producing infectious influenza B viruses in cell culture, the method comprising:

i) electroporating a population of Vero cells with eight plasmids comprising nucleic acid sequences corresponding to an influenza B virus genome encoding an attenuated, cold adapted, temperature sensitive influenza virus;

ii) culturing the population of Vero cells under conditions permissive for viral replication in the absence of helper virus at a temperature less than or equal to 35° C. and in serum free medium; and,

iii) recovering a plurality of infectious influenza B viruses.