SAN DIEGO, Calif.— A top health official calls it "very reassuring" news.

He says the U.S. now has more than enough smallpox vaccine to protect Americans if there is a bioterrorist attack – because testing has shown that watered down doses can still be effective.

Dr. Anthony Fauci of the National Institute of Allergy and Infectious Disease spoke at a conference in San Diego on Sunday.

Fauci says the diluted vaccine has been tried on more than 100 volunteers to see if it still works. He says the results show that a cache of 86 million doses that came to light in March contains enough to vaccinate everyone in an emergency.

The last smallpox case in the United States was in 1949, and routine vaccination stopped in 1972.