The invention relates to a method for producing the flu virus, that comprises:a) administering a flu vaccine to hens; b) collecting the eggs of the vaccinatedhens; c) triggering the embryogenesis process; d) infecting the embryonatedeggs by inoculating a flu virus in the allantoic cavity; e) incubating the infectedembryonated eggs in temperature and humidity conditions allowing the virusreplication; and f) collecting the allantoic liquid containing the virus.