Seasonal flu vaccination. Close_up of an influenza flu vaccine being injected into a patient磗 arm. The vaccine contains inactivated virions virus particles of strains of the influenza flu virus. When injected, the vaccine stimulates the body磗 immune system to produce antibodies, but without causing an infection. The immune system can then respond quickly against future infections by the flu virus. The flu vaccine is given to people at risk, such as the elderly and very young. The vaccine is provided annually because the flu virus can produce new strains that the body is not protected against. This vaccine is being used for the 2011 2012 season.