Now, the Bush administration is preparing to offer it to all 280 million Americans. The question being debated is not whether the general public should get it, but how fast and under what circumstances, according to three officials involved in the planning.

Experts don't know if the nation will ever be attacked with smallpox, which kills one-third of its victims. Eradicated from nature two decades ago, it is still feared as a bioterror agent. But the vaccine itself carries rare but serious risks, including death, complicating any decision to inoculate people absent a certain risk.

The Bush administration has yet to make final decisions or announce plans for what is called "pre-attack" smallpox vaccination. But administration officials say the consensus is to begin vaccinating those at greatest risk of encountering a highly contagious smallpox patient, such as hospital emergency room workers. That could total a half million people. Then the vaccine would be offered to non-hospital health workers, such as primary care doctors, and to police, firefighters and other emergency workers.

At some point after that, it would be offered to the general public.

"You start with one group and based on their potential risk, you keep expanding," one administration official said Thursday, speaking on condition of anonymity.

But it hasn't been decided how many people will get vaccinated during the first wave of shots, or how long it will take to offer them to everyone.

At issue, officials say, are important details such as who should get the vaccine while it's still an experimental drug, which requires a lengthy procedure to ensure that people understand and accept the risks. One option is to wait until the vaccine is fully licensed by the Food and Drug Administration ( news - web sites) before offering the vaccine widely, which could take a couple of years.

Another unanswered question is liability — how to compensate people injured by the experimental vaccine.

Made from a live virus, the vaccine itself is risky, particularly for people with certain skin diseases and weak immune systems. Studies from the 1960s suggest one or two people per million inoculated will die. About one in 1,000 will face complications, some serious, including a severe skin rash or encephalitis that may kill or cause permanent neurological damage.

Other issues are logistical, such as how states and cities prepare large vaccination clinics and train people to give the shots, and how to deal with people who get sick from the vaccine and the publicity likely to surround such an incident.

Those questions get more complicated as the number of people vaccinated increases.

Within the administration, some say Vice President Dick Cheney ( news - web sites)'s office is pushing for an aggressive policy, acting fast to inoculate much of the nation, though his office says Cheney is not taking sides.

Some health officials are counseling a slower approach, in which they could analyze the results of the first round of vaccinations before moving to the next.

Either way, the administration's direction represents a remarkable shift in thinking in a very short time, say experts both in and out of government. The reasons, they say, are practical, political and philosophical.

Immediately after last fall's anthrax attacks concentrated attention on bioterrorism, mass vaccinations were not considered because there wasn't enough vaccine: only 15.4 million doses in storage with another 40 million on order.

Since then, researchers have determined that by diluting the 15.4 million doses, 75 million people can be inoculated. A drug company found another 86 million doses in its freezer and donated them to the government. And the Department of Health and Human Services ( news - web sites) signed or expanded contracts for 209 million new doses, which should arrive early next year.

The new shipments bring a natural pressure to offer the vaccine. Newspaper editorials and leading voices, including Sen. Bill Frist, R-Tenn., a physician, have argued that people should be allowed to weigh the risks and decide for themselves whether to be vaccinated.

"What if you do have an attack and people die and you had something you could have given them, how do you answer that?" a second administration official said.

On top of that looms possible war with Iraq. After the disease was declared eradicated, all smallpox was supposed to be destroyed except for samples kept in special labs in Atlanta and Moscow. But experts fear that rogue states including Iraq have it.

"We're very worried about Iraq," said Dr. D.A. Henderson, a top HHS bioterrorism adviser. "Why is Saddam Hussein ( news - web sites) pushing ahead with weapons of mass destruction if at some point he is not going to use them? It's certainly got to be a factor in all of this."

Another factor: Officials are realizing how complicated it will be to vaccinate large numbers of people quickly after a smallpox attack. This week, federal officials gave states guidelines for mass vaccinations after an attack; most cities are far from ready.

If there were an attack, mass vaccinations would be significantly easier if many people are already protected. Some argue aggressive vaccinations might even deter an attack.

A decision had been expected by the end of September, but officials now say it's not expected until next month.

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