President Bush is expected to urge smallpox vaccinations for 500,000 emergency workers most likely to be exposed to the virus in a bioterror attack and order 500,000 military personnel to get the shots, White House officials said Wednesday.

Eventually, the vaccine will be made available to all Americans, though the government will probably not encourage them to get it, according to senior officials.

Bush does not plan to announce his policy for at least a week, and details could change, said the officials, who spoke on condition of anonymity.

Barring a change, Bush plans to order vaccines for a half-million military personnel. On the civilian side, where his thinking is less certain, he is inclined to accept recommendations to order vaccines for about 500,000 medical and emergency-response workers.

Questions about who should be offered the vaccine, which carries risks including possible death, and whether to recommend it or just make it available, have occupied federal health officials and now the White House for months.

Smallpox was wiped out worldwide two decades ago, but experts fear the contagious, often fatal disease could return through a terror attack or war. Routine vaccinations ended in the United States in 1972, meaning nearly half the population is without any protection from the virus. Health officials aren't sure whether those vaccinated decades ago have residual protection.

The new vaccine would be offered in stages, beginning with those most likely to encounter a smallpox patient. That includes people on state response teams, who would investigate suspicious cases, and people who work in hospital emergency rooms.

In a second phase of vaccination, the shot could be offered to other health care workers and emergency responders such as police, fire and emergency medical technicians. Federal officials probably will recommend the shot for these roughly 10 million people, too.

Federal officials plan to work with states and hospitals to identify who most needs to be inoculated. One official said the administration wants to give states flexibility to whittle down the number for whom the shot is recommended.

Plans are not complete for how the vaccine would be offered to the public, but officials do not plan to recommend it for Americans who have no particular risk of exposure.

That's because of the risk of the vaccine itself. Based on studies from the 1960s, experts estimate that 15 out of every 1 million people vaccinated for the first time will face life-threatening complications, and one or two will die. Reactions are less common for those being revaccinated.

Using these data, vaccinating the nation could lead to nearly 3,000 life-threatening complications and at least 170 deaths.

But the administration has concluded that the government cannot make it available to some people and not others who may want it, said a third official.

There are several possibilities. Most of the vaccine on hand is not yet licensed by the Food and Drug Administration, and what is licensed will be saved for people in stages one and two. For the general public, federal officials could simply wait until more vaccine is licensed, probably in early 2004. They also could enroll interested people in experimental trials and offer the shot that way.

Meanwhile, states are working on their own smallpox plans, due next week at the Department of Health and Human Services. Those plans are to deal with how states would inoculate mass numbers of people if the smallpox virus were to be released.