The vaccine, ACAM2000, is intended to innoculate people at high risk of exposure to smallpox, a highly contagious disease. The FDA said the vaccine also could be used to protect individuals and populations during a bioterrorist attack.

"The licensure of ACAM2000 supplements our current supply of smallpox vaccine, meaning we are more prepared to protect the population should the virus ever be used as a weapon," said Dr. Jesse L. Goodman, director of FDA's Center for Biologics Evaluation and Research.

Goodman said the vaccine is made using modern cell culture technology that would allow for speedy manufacturing if large quantities were needed quickly.

ACAM2000 is made by Acambis Inc. of Cambridge, England and Cambridge, Massachusetts. The federal Centers for Disease Control and Prevention already has stockpiled 192.5 million doses of the vaccine.

The U.S. ended routine vaccination against smallpox in 1971, and world health authorities declared the disease eradicated from the wild in 1980. The last known case was reported in Somalia in 1977.

But after the terrorist attacks of Sept. 11, 2001, concern arose that smallpox and other infections could be engineered as weapons. That led to the stockpiling of certain vaccines in case they ever are needed — and to vaccinate some military personnel and health care workers.