A U.S. government health official says the nation will soon have enough vaccine to protect everyone if there is a bioterrorist attack using smallpox. The U.S. government could stretch the limited stores by diluting the vaccines on hand.

The head of the U.S. government's National Institute of Allergy and Infectious Diseases, Dr. Anthony Fauci, says experiments to dilute existing smallpox vaccine stocks and an order for more vaccine from a British company means there will be enough doses for the entire U.S. population.

Authorities have long considered smallpox a leading potential bioterrorist weapon, even though the virus was eradicated in the 1970s. The United States and Russia are believed to have the only existing stocks locked up in research facilities, but officials do not discount the possibility that some research samples may have been stolen or never destroyed in the first place.

After the recent anthrax scare, Dr. Fauci's agency undertook studies to see if the country's 15 million smallpox vaccine doses could be multiplied by as much as ten times by dilution and still be effective. He told the meeting of the American Association for the Advancement of Science in Boston that a study on volunteers shows that dilution is effective.

"Very likely by the beginning of March, the data will be officially out, but I can tell you that the study was extremely successful," he said. "The precise details of this will come out when the data are officially presented, but it was a very successful experiment."

This means that 15 million stored doses of smallpox vaccine can be turned into 75 million doses if they are diluted by a ratio of one-to-five. To protect the other 200 million Americans, the U.S. government has ordered 200 million doses from a British pharmaceutical company for delivery by the end of this year. Officials would call on these first in the event of a bioterrorist attack.

But the vaccine, which uses a live vaccinia virus to raise a person's immunity, has strong side effects. It kills one or two people per million and causes complications such as encephalitis in hundreds more. So U-S officials do not plan a mass preventative inoculation campaign unless a terrorist smallpox threat becomes real.

Dr. Fauci says his agency is developing a new vaccine that would not cause such complications.

"The third generation is something that we're just now launching," he said. "The ultimate goal of the third generation is to have an essentially non-toxic vaccine that you wouldn't hesitate for a moment to vaccinate anyone with."

Smallpox is just one of several microbes considered to be possible bioterrorist weapons. A biological weapons expert told the science conference that they are very hard to defend against. Veterinarian David Franz of the Southern Research Institute says any program to counter the threat must consider discouraging the use of such weapons.

"There are no really good, simple technical solutions to this problem. I think it's useful to think about changing intent among those who might use biological agents against us," he said.

Dr. Franz says an important factor working against bioterrorists is that biological weapons are very difficult to deploy.