Last Updated:
16 May 12

Smallpox vaccine is made from a virus called vaccinia, another “pox”-type virus related to variola virus, the virus that causes smallpox. But smallpox vaccine does not, and cannot, cause smallpox. This vaccine was prepared on the skin of calves. The calves’ lymph was purified, concentrated, freeze-dried, and stored in a freezer until recently.

This same vaccine has been administered to many millions of Americans, including Service Members during World War I, up until 1990.

The Defense Department resumed smallpox vaccinations for selected service members in December 2002.

The smallpox vaccine contains a living virus called vaccinia. After vaccination, this live virus grows in the skin and stimulates a person’s immune system to develop antibodies and cells that 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, skin rash, or fatigue) result from the vaccine virus or from the vaccinated person’s reaction to the virus.

What if I have already been exposed to smallpox?
Smallpox vaccine is usually given before a smallpox outbreak to prevent infection, but, if a smallpox outbreak happens, the vaccine can be helpful if given quickly after exposure.

If administered within 3 days of exposure, it can prevent death or significantly modify smallpox in many people.

Vaccination 4 to 7 days after exposure may offer some protection from death or may modify the severity of disease.

Is it possible for me to get smallpox from my vaccination?
No. In short, smallpox vaccine is named after the disease it prevents, not after the virus it contains.

The smallpox vaccine does not contain smallpox virus and cannot spread or cause smallpox. However the vaccine does contain another live virus called vaccinia. Because the vaccinia virus is live, it can spread to other parts of the body or to other people from the vaccination site. This can be prevented through proper care of the vaccination site (e.g., hand washing and careful disposal of used bandages).

Studies show that the first dose of the vaccine offers solid protection against smallpox for about 5 years, with decreasing immunity thereafter. Solid protection after revaccination lasts about 10 years.

I was vaccinated before – do I need to be vaccinated again?
The protection offered by vaccination against smallpox falls over time. Solid protection lasts for 3 to 5 years after your first vaccination (10 years after revaccination). Partial protection lasts longer, but people need to be revaccinated if too much time has passed.