Influenza vaccine boxes and pre-filled syringes. This brand, the Inactivated Influenza Vaccine (Split Virion) BP, is one of the recommended vaccines for the northern hemisphere in the 2005/6 flu season. It contains inactivated virions (virus particles) of three strains of the influenza (flu) virus. When injected, the vaccine stimulates the body's 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 marketed in the UK by Sanofi Pasteur MSD.