"In an emergency, we have enough vaccine for every American now," Gerberding told a news briefing.

CDC, the Department of Health and Human Services ( news - web sites) and the National Institutes of Health ( news - web sites) are preparing to advise President Bush ( news - web sites) on who should be vaccinated so that the country is prepared for a smallpox attack.

They issued advice last week to states about what to do to prepare for mass vaccinations if an actual attack occurred.

The officials say they are struggling with the decision because current smallpox vaccines are crude and dangerous. They are left over from the vaccination programs of the 1960s and 1970s and have a high incidence of side effects.

When smallpox was eradicated from the world's population in 1979, officials gladly stopped vaccinating the public. "We stopped using this vaccine when this disease was eradicated because it was dangerous," Gerberding said.

For every 1 million people vaccinated, one to two will die from side effects of the vaccine, she said, and 15 will develop life-threatening complications such as encephalitis. But in a smallpox outbreak, a third of the victims may die.

"The only way...we can be sure not to do harm is if there is no vaccination and there is no attack," she said.

NO PROVEN DRUG TREATMENT

Bioterrorism experts have said for years the US is vulnerable to a biological attack, a risk that went up with last year's Sept. 11 attacks against Washington and New York and the anthrax letter attacks that followed.

Smallpox is considered a potential weapon because it is known the Soviets had stocks of the virus, and officials fear other countries and groups may have it as well. Unlike anthrax, smallpox spreads from person to person and there are no drugs to treat it although one drug, cidofovir, is being tested.

The government now wants to pre-vaccinate health and emergency workers who would have to help victims of any smallpox attack, but is trying to balance risks and benefits.

Gerberding estimates about 7 million healthcare workers, including emergency room teams, could be vaccinated ahead of time. Each community would probably need to have some police, emergency and ambulance workers protected as well.

"It could involve about 10 million people, we think, when we add up all the numbers," Gerberding said.

And if there were an attack, experts are inclined to recommend the vaccine be offered to the general public. "One strategy is to let people make up their own minds," she said.

Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases ( news - web sites), said 15 million doses of the 30-year-old DryVax vaccine are on hand, along with 75 million to 80 million doses that vaccine maker Aventis Pasteur found warehoused and offered to the United States.

These could be diluted and still work, he said.