The invention relates to a method for producing flu virus according to which: a) immunizing a hen by administering a flu vaccine to the hen, b) triggering embryogenesis in one or more eggs of the immunized hen, c) infecting the one or more embryonated eggs by inoculating a flu virus into the allantoic cavity of the eggs, d) incubating the one or more infected embryonated eggs under temperature and humidity conditions that allow replication of the virus, and e) harvesting the allantoic fluid of the one or more incubated eggs containing the virus.
