Modified viral strains and method for improving the production of vaccine seeds of influenza virus

1. A modified influenza A/PR/8/34 virus, whose quantity of HA-NA (hemagglutinin and neuraminidase) glycoproteins on the surface is greater than 550 glycoproteins for a virion of 100 nm in diameter, including a mutated PB1 gene of SEQ ID NO: 1 coding for a PB1 protein having at least two specific amino acid modifications selected from the group consisting of: 188 (K→E), 205 (M→I), 212 (L→V), 216 (S→G), 398 (E→D), 486 (R→K), 563 (I→R), 576 (I→L), 581 (E→D), 584 (R→Q), 586 (K→R), 617 (D→N), 621 (Q→R), 682 (V→I) and 691 (R→K), wherein the modified influenza A/PR/8/34 virus is reassortant including the HA and NA genes of another influenza virus.

2. The modified influenza A/PR/8/34 virus according to claim 1 , wherein it includes a mutated PB1 gene of SEQ ID NO: 1 coding for a PB1 protein with at least both amino acid modifications 563 (I→R) and 682 (V→I).

3. The modified influenza A/PR/8/34 virus according to claim 1 , wherein it includes the PB 1 gene of another strain of influenza A virus whose quantity of HA-NA surface glycoproteins is greater than 550 glycoproteins for a virion of 100 nm in diameter.

4. The modified influenza A/PR/8/34 virus according to claim 1 , wherein it includes the PB1 gene of H3N2 influenza virus having the sequence of SEQ ID NO: 2.

5. The modified influenza A/PR/8/34 virus according to claim 1 , including the HA and NA genes of an influenza virus selected from the viruses having the H3N2, H2N2, H1N2, H5N2, H5N1, H7N7, H9N2 and H3N1 subtypes.

6. A method for producing HA-NA vaccine glycoproteins of influenza virus, wherein a modified influenza virus according to claim 1 including the HA and NA genes coding for said HA-NA vaccine glycoproteins is amplified in eggs or in cells.

7. The method for producing HA-NA vaccine glycoproteins of influenza virus according to claim 6 , wherein the HA-NA vaccine glycoproteins are selected from the glycoproteins of viruses having the H3N2, H2N2, H1N2, H5N2, H5N1, H7N7, H9N2 and H3N1 subtypes.

8. A method for increasing the quantity of HA-NA glycoproteins on the surface of an influenza A/PR/8/34 virus, wherein it includes the modification of the PB1 gene of SEQ ID NO: 1 of said influenza A/PR/8/34 virus in order to introduce into the PB1 protein coded by said PB1 gene at least two specific amino acid modifications selected from the group consisting of: 188 (K→E), 205 (M→I), 212 (L→V), 216 (S→G), 398 (E→D), 486 (R→K), 563 (I→R), 576 (I→L), 581 (E→D), 584 (R→Q), 586 (K→R), 617 (D→N), 621 (Q→R), 682 (V→I) and 691 (R→K), wherein the influenza A/PR/8/34 virus is reassortant including the HA and NA genes of another influenza virus.

9. The method for increasing the quantity of HA-NA glycoproteins on the surface of an influenza A/PR/8/34 virus according to claim 8 , wherein it includes the modification of the PB1 gene of SEQ ID NO: 1 of said influenza A/PR/8/34 virus in order to introduce into the PB1 protein coded by said PB1 gene at least the two specific amino acid modifications 563 (I→R) and 682 (V→I).

10. The method for increasing the quantity of HA-NA glycoproteins on the surface of an influenza A/PR/8/34 virus according to claim 8 , wherein it includes the replacement of the PB1 gene of SEQ ID NO: 1 by the PB1 gene of another influenza A virus strain whose quantity of HA-NA surface glycoproteins is greater than 550 glycoproteins for a virion of 100 nm in diameter.

11. The method for increasing the quantity of HA-NA glycoproteins on the surface of an influenza A/PR/8/34 virus according to claim 10 , wherein it includes the replacement of the PB1 gene of SEQ ID NO: 1 by the PB1 gene of an H3N2 influenza virus having the sequence of SEQ ID NO: 2.

12. The method for increasing the quantity of HA-NA glycoproteins on the surface of an influenza A/PR/8/34 virus according to claim 1 , wherein the A/PR/8/34 virus is a reassortant virus including the HA and NA genes of an influenza virus selected from the viruses having the H3N2, H2N2, H1N2, H5N2, H5N1, H7N7, H9N2 and H3N1 subtypes.

13. A method for increasing the immunogenicity of a dose of vaccine including HA-NA glycoproteins of influenza virus by raising the NA/HA ratio of the glycoproteins produced, wherein the glycoproteins are produced by amplification in eggs or in cells of a modified influenza virus according to claim 1 .

14. A method for increasing the immunogenicity of a dose of vaccine including HA-NA glycoproteins of influenza virus by raising the NA/HA ratio of the glycoproteins produced, wherein the glycoproteins are produced by amplification in eggs or in cells of a modified influenza virus according to claim 5 .