Dryvax, the only currently licensed smallpox vaccine, and other smallpox vaccines that would be made available as investigational products in the event of an emergency contain a living virus (the vaccinia virus). Following vaccination, this living virus grows in the skin and stimulates a person's immune system to develop antibodies and cells in the blood and elsewhere that can then help the body fight off a real smallpox infection if the person is ever exposed. Most of the common side effects of the vaccine (like fever and the skin rash and weakness) occur from the growth of the vaccine virus itself or from the vaccinated person's reaction to the virus. Many of the rare, most serious side effects, occur in people whose immune systems fail to effectively fight off the virus. These types of adverse events, though unwelcome and unwanted, are due to the nature of the vaccinia virus itself and are expected to occur in a certain number of individuals (see above). The occurrence of such serious, life-threatening events is expected with use of currently available smallpox vaccines and is part of the reason that anyone receiving a smallpox vaccine, even licensed Dryvax, should be informed of the potential risks | 
