White House officials are considering how many Americans to vaccinate against smallpox, but that's just one of many issues complicating planning for what officials see as the scariest bioterror scenario.

Smallpox, a highly contagious and fatal disease, was declared eradicated worldwide in 1980. The United States, which hasn't seen a case in decades, ended routine immunizations in 1971. But experts fear samples of the virus could be in the hands of hostile nations or terrorist groups.

Vaccinations effectively protect against smallpox, but the shots carry serious, sometimes fatal, side effects. Federal officials are weighing the risk of the side effects against the risk of a smallpox attack, trying to decide how many people need to be vaccinated in advance. Vaccination is also effective if delivered within a few days of being exposed to the disease.

Health and Human Services Secretary Tommy Thompson said Tuesday he has sent his recommendation for how many people to vaccinate to the White House and expects a final decision soon. He would not say what he recommended; options range from limiting vaccines to a small group of health care workers to offering it to all Americans.

Meantime, HHS officials are sorting through a vast array of issues associated with the vaccine. A newly convened panel of HHS bioterrorism advisers heard a rundown Tuesday. Among them:

--Recovery time. After the vaccination, many people are sick for several days. Those vaccinated also may spread smallpox virus to others -- possibly killing them -- if they do not keep the vaccination site tightly covered. If a large number of hospital workers get vaccinated, will they get time off work? Will large numbers of workers all be off at the same time? Who pays for the lost hours?

--Liability. If someone dies or has a serious illness after getting the vaccine, who is legally liable? The new vaccine that would be used has not been tested in humans, so patients must sign consent forms before they are injected. But those forms may not protect the government from liability. And what liability might an employer or adviser who recommends the vaccine have?

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

--Reporting. If someone does have a reaction to the vaccine, whom do they call?

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

Dr. D.A. Henderson, Thompson's top bioterrorism adviser, led the World Health Organization's smallpox eradication campaign. He said he never encountered anywhere near as many "issues and problems" in decades of work with the global program.

"This is a whole different environment," he told the Secretary's Council on Public Health Preparedness.

Vaccinations also can be given after people are exposed to the disease, so planning is under way for mass vaccinations should an attack occur. Henderson also said many American cities are vastly unprepared to deliver those shots.

Many communities have yet to figure out where to set up clinics, who will administer the shots and how to educate masses of people about the health risks that the vaccine carries, experts told the council.

"We're lagging behind, there's no question on that," Henderson said. "We're going to have to push very hard."

Among the challenges are where would people go for vaccinations, and are these clinics prepared to handle a crush of patients. Also, what health care providers would be available to run the vaccinations and how can officials efficiently inform large numbers of people about the risks and benefits of an experimental drug.