The currently licensed smallpox vaccine, which consists of a laboratory strain of vaccinia virus, is highly effective in preventing infection. Medical experts believe the vaccine may lessen the severity of, or even prevent, illness in unvaccinated people if given within 4 days of exposure to the virus. The smallpox vaccine helps the body develop immunity to smallpox. The vaccine is made from a "pox"-type virus related to smallpox. The smallpox vaccine contains live vaccinia virus-unlike many other vaccines that use killed virus. The vaccine does not contain the smallpox virus and cannot transmit smallpox. Learn more about the smallpox vaccine from CDC.
Few data exist showing just how long vaccinia vaccines protect people against smallpox infection. Therefore, those vaccinated against the smallpox virus before 1972 may be susceptible to the disease. Military and other high-risk groups (for example, scientists who work with vaccinia and other orthopoxviruses related to Variola major) have been getting the vaccine since the United States stopped routine smallpox vaccinations in 1972.