The first step in developing a flu vaccine each year is predicting which strains, or versions, of the virus are likely to be the most common. Three "seed" strains are developed; these are versions of the virus that are not harmful. But the viruses can't multiply on their own: they need a host to live in.
That's why flu vaccines are made inside fertilized chicken eggs -- a method that was developed 50 years ago. The shell is cracked so that the flu virus can be injected into the egg. The egg is resealed, the embryo inside becomes infected, and it can then be removed and purified to produce a vaccine. Each egg incubates just one of the three seed strains. But after harvesting, all three strains are blended together.
It's a labor-intensive process that takes at least six months. Millions of specially purified fertilized chicken eggs are needed (the eggs we buy in the grocery store aren't fertilized). They must be delivered at regularly scheduled intervals from February to August of each year, and there is no way to increase production in the case of an unexpected epidemic once the order for eggs has been placed.
That's why scientists would like to switch to growing the flu virus in lines of cells taken from humans, monkeys, dogs, or even insects. Using cells would enable them to cut back on the time needed to produce the vaccines. It's also much easier to scale up cell cultures to produce more vaccine in the case of a flu epidemic. Some companies are already producing vaccines for polio and smallpox using monkey kidney cells.