The United States is expected have enough vaccine against smallpox to protect everyone in the country if terrorists unleash the deadly virus on U.S. soil. The existing stores can be stretched by dilution, and a drug company has discovered millions more doses in storage.

The U.S. Health and Human Services Department says a study shows that the existing supply of 15 million smallpox doses can be multiplied to 75 million by dilution without losing potency. Together with the planned government purchase of 209 million more doses from a British company, this would be sufficient to vaccinate the entire nation in the event of the release of smallpox by bioterrorists.

Expanding the U.S. smallpox vaccine supply has been part of the country's bioterrorism preparedness plan for several years, but became a higher priority after the anthrax scare late last year.

The new study tested the diluted vaccine on nearly 700 young adults who had never been inoculated for smallpox because the disease was eradicated in the late 1970s. The preliminary results were announced last month at a Boston science meeting by the head of the U.S. government's National Institute of Allergy and Infectious Diseases, Dr. Anthony Fauci. "The study has been completed," he said. "The study was extremely successful in the sense of a very high 'take rate.' It was a very successful experiment."

The 'take rate' is the frequency with which blister-like sores appear at the injection site. It is an indirect measure of the vaccine's effectiveness. In the study, more than 97 percent of participants had such a blister with the diluted vaccine.

In addition, the U.S. drug company Aventis-Pasteur has found between 70 and 90 million doses of smallpox vaccine sitting in its freezers. The company is determining whether the vials are still potent, and health officials say the government is discussing whether to purchase them.

But the vaccine 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 preventive inoculation campaign unless a terrorist smallpox threat becomes real.

But Boston University medical professor William Bicknell said that policy is wrong. Writing in The New England Journal of Medicine, he said inoculating after an attack is unlikely to contain the disease and could cost more lives.

Dr. Fauci said his agency is developing a new vaccine that would not cause such complications. "The ultimate goal is to have an essentially non-toxic vaccine that you wouldn't hesitate for a moment to vaccinate anyone with," he said.