What are vaccines for influenza made from?

Vaccines for influenza are made of from fertilized chicken eggs. It begins with each egg being injected with one to four strains of flu virus that will dominate the flu season. The eggs give a good environment for the virus to multiply. The contents of the eggs then undergo a complex process involving the inactivation and break-up of the virus so that it can stimulate an immune response without actually causing the flu. The virus-containing fluid is harvested from the eggs. For flu shots, the influenza viruses for the vaccine are then inactivated (killed), and virus antigen is purified.