SAN DIEGO -- By diluting long-stored doses, the United States now has more than enough smallpox vaccine to protect everyone in case of a bioterrorist attack, a top health official said Sunday.

Testing some of the 86 million doses of vaccine that came to light last March shows that they can be watered down and still offer potent protection against smallpox.

Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said Sunday that the diluted vaccine has been tried on more than 100 volunteers to see if it still works. The results show this cache alone contains enough to vaccinate everyone in an emergency.

"This is very reassuring," Fauci said. He spoke at an infectious disease conference in San Diego sponsored by the American Society for Microbiology.

The last smallpox case in the United States was in 1949, and routine vaccination stopped in 1972. However, the Soviet Union stockpiled tons of the virus during the Cold War, and experts worry some of that might be used as a weapon by terrorists.

Many experts assumed that vaccination would begin again once new smallpox cases were discovered. But last week, government officials said they are planning to offer smallpox vaccine before such an attack to all 280 million Americans.

However, that would involve distributing vaccine that is still being made for the government. The first of that supply should arrive early next year from Acambis Inc., which has been contracted to make 209 million doses.