In order to mass produce a vaccine the seed virus must be able to grow efficiently in chicken eggs. To achieve this, a nonpathogenic strain that grows well in eggs and the circulating strain are injected into fertilized chicken eggs, (Hood, 2006). The eggs are then incubated to speed up the viruses growth. As the viruses multiply, their genes mix, creating up to 256 combinations of eight genes, (Hood, 2006). After the eggs are incubated, the resulting viruses are meticulously screened for a virus that has the for the desired HA and NA subtypes and the six genes that allow the virus to thrive in eggs. According Hood the process to generate a seed virus, "could take two weeks optimistically, but more realistically one to two months." (2006). After a viable seed virus is found, it is injected other eggs to create a seed virus stock. This process is repeated until a seed virus is created for all the strains included in the vaccine. The seed viruses are then sent to manufacturers across the country.
Mass Production and Refinement.
When the manufacturers receive the seed viruses they inject it into eggs on a much larger scale to mass produce each strain. The eggs are incubated for multiple day to give the virus time to replicate. After the incubation is complete, the virus containing liquid in the eggs is extracted, (FDA, 2013). The viruses that will be in the inactivated vaccine (flu shot) are killed, while viruses that will be in the live attenuated vaccine (nasal spray) are weakened significantly, (CDC, 2015). Before the strains are mixed to form the quadrivalent vaccine they are be tested by the manufacturer and the FDA for potency and safety. If the viruses meets the stringent FDA requirements they are mixed together to formulate the final vaccine.
Final Testing.
The final vaccine mixture is tested again before it is put in a standard dosage container. The flu shots are loaded into vials and syringes.
