Helper-free rescue of recombinant negative strand RNA virus

A method for producing influenza virus particles comprising producing in a cell all of the genomic RNA of an influenza virus by expressing from DNA introduced into the cell at least one or more of genomic RNA or cRNA of the influenza virus and producing in the cell RNA dependent RNA polymerase and nucleoprotein and assembling in the cell the influenza virus, said cell being free of helper virus, wherein said cell is selected from Vero cells and other cells which are deficient in interferon activity and capable of supporting growth of said virus.

A method for producing influenza virus particles for preparation of a vaccine, comprising:

• growing a reassortant influenza virus to produce virus particles and recovering said produced virus particles, wherein said reassortant influenza virus is produced in cultured cells in the absence of helper virus by introducing into said cultured cells expression vectors to produce in said cells the genomic vRNA or antigenomic vRNA (cRNA) segments of a reassortant influenza virus and producing in said cells a nucleoprotein and an RNA dependent RNA polymerase so that RNP complexes containing the genomic vRNA segments of said reassortant influenza virus are formed,
• wherein said expression vectors encode at least 4 different vRNA (or cRNA) segments and wherein each said different vRNA segment is from a single influenza virus strain, wherein all said different vRNA segments are not from the same influenza virus strain, wherein said different vRNA segments include an HA and an NA vRNA segment both of which are from the same influenza virus strain, and wherein said different vRNA segments are selected from PB2, PRI, PA, HA, NP, NA, M and NS.