Influenza vaccine manufacturers use the primary seed virus strains to produce vaccines that match the strains of the respective virus subtypes that are likely to circulate during the next influenza season. First, the virus strains are separately propagated in embryonated chicken eggs to produce large amounts of the viruses, called the working seed virus stock. This secondary seed virus serves as a source for the production of virus batches on a large, industrial scale. In this process, millions of eggs with 10 to 11 day-old embryos are automatically inoculated with a small amount of secondary seed virus. After an incubation period of three days at a temperature of 32 to 36°C, the liquid that surrounds the embryos (allantoic fluid) and contains huge amounts of the virus particles is extracted from the eggs. The viruses are then purified and inactivated so that they can no longer replicate. In the last step, the different virus preparations are mixed and filled into syringes. Obviously, live vaccine (LAIV) viruses are not inactivated, since they are weakened or attenuated viruses that still need to replicate in the vaccinated person.