SATURN STILLS / SCIENCE PHOTO LIBRARY SATURN STILLS / SCIENCE PHOTO LIBRARY
Influenza vaccine in a pre-filled syringe. This vaccine contains inactivated virions, or particles, of four 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 only given to people at risk, such as the elderly and very young. The vaccine is provided annually because of the flu virus' ability to produce new strains, which the body is not protected against.
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