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.
Only two approved U.S. and Russian labs keep known stockpiles of smallpox, which the CDC considers among the greatest potential threats to public health.
"Smallpox could be a particularly dangerous biological threat to us that would kill or debilitate a high percentage of the population," said Dr. W. Craig Vanderwagen, a rear admiral and assistant secretary for preparedness and response at the Department of Health and Human Services.
Smallpox is caused by the variola virus, which spreads through close contact with infected individuals or contaminated objects. There is no FDA-approved treatment for it.
The new vaccine is derived from the nation's old smallpox vaccine, called Dryvax, which is no longer made, although there are leftover supplies. ACAM2000 is made using a pox virus called vaccinia, which is related to but different from the virus that causes smallpox.
It contains live vaccinia virus, the FDA said, and works by causing a mild infection that stimulates an immune response that effectively protects against smallpox without actually causing the disease.