The plan would begin vaccinations for those at the greatest risk of contacting a patient with the highly contagious disease. That includes infectious disease specialists and emergency room personnel, including doctors, nurses, technicians, even security officers working at hospitals and clinics, according to officials familiar with the administration's planning.

This first group probably will include more than 1 million people, one official said, although precise numbers won't be known until states are given guidelines and determine how many people they cover.

The vaccinations carry risk of serious side effects, including death, so authorities estimate that only about a third to a half of those offered vaccinations will take them. In the end, they believe that several hundred thousand – maybe half a million – will wind up getting the vaccinations.

Officials cautioned that plans still could change. They expect to announce a decision by the end of the month.

Smallpox hasn't been seen in this country in decades, and routine vaccinations ended in 1971. In 1980, the disease was declared eradicated from the globe, but experts fear hostile nations or terrorist groups might acquire the virus and unleash it in a bioterror attack.

The plan on the table would vaccinate people in stages, based on risks they face.

In the second stage, vaccinations would be offered to other health care workers, including those in private practice and others who work in hospitals but are not at direct risk, officials said. At some point, emergency personnel, such as police and firefighters, also would be offered the vaccinations.

"Ultimately all health care workers will be vaccinated," said one official involved in the planning, speaking on condition of anonymity.

Eventually, the vaccine will be offered to the general public, he said, adding: "It will take years for that to happen."

In addition, plans are being put into place to quickly vaccinate everyone, should that become necessary.

Officials will evaluate side effects of the vaccine before moving to each new group, he added.

For federal officials – including President Bush, who will have the final say – the challenge is balancing the risk of the vaccine, which is known, with the risk that smallpox might return, which is not known.

Top health officials say they have to assume that would-be terrorists have the virus and might use it.

"My gut tells me you have to assume the worst right now with bioterrorism," Health and Human Services Secretary Tommy Thompson said in a recent interview.

Using similar logic, Sen. Bill Frist, R-Tenn., a leader on bioterrorism questions, said last month that all Americans should be allowed to decide for themselves whether to get the vaccine.

Others argue that any specific person faces a very low risk of ever seeing the disease but faces a certain risk from the shot. About one in 1,000 people vaccinated will face complications, some serious, and the vaccine will kill one or two out of every million people who get it.

Using this logic, in July a federal advisory committee recommended limited vaccinations. They recommended vaccinating small response teams for each state and a few caregivers at designated hospitals – probably 10,000 to 20,000 altogether.

"The likelihood of any one health care worker coming into contact with a case of smallpox, even in the likelihood of an outbreak, would be exceedingly small," said John Modlin of Dartmouth Medical School, who chairs that committee.

People can be vaccinated effectively against smallpox for several days after they are exposed; so an urgent round of vaccinations would begin as soon as a patient had been diagnosed.

As they plan for vaccinations, federal officials face a host of complex issues:

–Recovery time. After vaccination, many are sick for several days. Will large numbers of hospital workers all be home sick at the same time? Who pays for the lost hours?

–Liability. If someone dies or gets very sick, who is liable? Thompson has suggested the government would shoulder legal liability, as it does with other vaccines.

–HIV/AIDS. People who have weakened immune systems, notably people with HIV, should not be vaccinated because they are at high risk for severe side effects. Not everyone who is HIV-positive knows of the condition. Should people be tested for HIV before vaccination? Should it be mandatory? Who would have access to the test results?

–Administering the vaccine. Each vial has 500 doses because so little vaccine is needed for each person. Once the vial is opened, it lasts only for about two weeks. Would people being vaccinated all have to come to the same place?

On the Net:

Centers for Disease Control and Prevention smallpox page: www.bt.cdc.gov/Agent/Smallpox/SmallpoxGen.asp