Methods for the propagation of modified influenza viruses in embryonated eggs

A method for vaccine production comprising:

• (a) propagating in an embryonated chick egg six to nine days old a genetically engineered, attenuated influenza virus, in which the genome of the attenuated influenza virus encodes a truncated NS1 protein of between 90 and 130 amino acid residues of the first N-terminal 90 to 130 amino acid residues of an NS1 protein of the same or a different influenza virus strain, so that the genetically engineered, attenuated influenza virus expresses the truncated NS1 protein and has an impaired interferon antagonist phenotype, wherein the N-terminal amino acid of the NS1 protein of the same or a different influenza virus strain is 1, and wherein the influenza virus is not influenza C virus;
• and
• (b) collecting progeny virus,
• wherein the virus is grown to sufficient quantities and under conditions that are free from contamination, such that the progeny virus is suitable for formulation into a vaccine.