At least 20 percent of Americans and possibly many more will not be able to receive smallpox vaccinations because their underlying medical problems make the vaccine unsafe, health officials said Thursday.

The assessment is one of several pieces of evidence suggesting that the federal plan to vaccinate about 500,000 hospital staff may not go smoothly or quickly. The prediction came as a volunteer panel that shapes federal vaccination policy wrapped up a two-day meeting in Atlanta.

The plan to vaccinate approximately 100 staff members each in 5,000 hospitals is intended to create a group that would care for the first victims of a bioterrorist smallpox attack. But those attending the meeting of the Advisory Committee on Immunization Practices, including academic medical experts and representatives of state health departments, said a number of barriers stand in the way.

The hurdles include the large number of medical conditions that could trigger severe side effects; liability protection for vaccine manufacturers; uncertainty over compensation payments for victims of side effects; and the ability to build a national patient-tracking and record-keeping system that must be completed before the first vaccinations begin.

In addition, health officials have begun to worry that publicity about side effects, which range from rashes and scarring to encephalitis and death in rare cases, may deter health care workers from volunteering to take the vaccine.

"If we really educate people about the risks, we may find people saying that they would rather wait and see," said Dr. Guthrie Birkhead of the New York State Department of Health.

No decision has been made regarding how many Americans, if any, will be vaccinated against a possible future attack. A ruling by the White House is expected within weeks.

The major federal health agencies --- the Centers for Disease Control and Prevention, National Institutes of Health and Department of Health and Human Services --- have outlined options for the White House that range from 500,000 hospital staff to 10 million health care workers and first responders to the entire population. On Wednesday, the advisory committee endorsed the 500,000-person recommendation.

That group should include not only doctors and nurses in emergency rooms and intensive care units, but radiology technicians, respiratory therapists, surgeons, specialists in eye and skin disorders, lab and housekeeping staff and security guards, the committee said.

The committee also added a list of conditions that could disqualify people from receiving the vaccine. No one should be vaccinated, the group said, if she is pregnant or intends to get pregnant shortly; has an impaired immune system from leukemia, long-term drug therapy, or transplants; or is infected with HIV, the virus that causes AIDS, or lives with someone infected.

Vaccinations also should not be given to anyone who has ever been diagnosed with eczema or atopic dermatitis, two broad categories of scaly rash disorders, or lives with anyone who has; or if the potential recipient or anyone in his or her household is having an outbreak of psoriasis, shingles or severe acne, or other breaks or wounds in the skin.

Skin disorders alone are expected to rule out 20 percent of potential recipients. That prediction is based on estimates from dermatologists, combined with data from a recent vaccine trial at St. Louis University in which one of every five volunteers was disqualified, said committee chairman Dr. John Modlin.

Privately, some meeting attendees put the number of potential recipients who would be ineligible under the panel's rules at 35 percent or higher. That would change, they added, if an attack began --- because the risk of actually contracting smallpox, a disease that kills one-third of victims and disables another third, would outweigh the risk of side effects.

Once the White House approves a plan, state health departments will have 60 days to meet with hospitals, recruit volunteers and design their vaccination campaigns. Each state will be allowed to send five staff members to the CDC for a week of training.

But the vaccinations cannot begin until a nationwide tracking network --- comprising doctors and other health workers and hotlines and computerized records --- is assembled. In addition to hardware, the network will need to recruit roughly three staff members at every hospital with a vaccinated team, plus more than 100 at federal agencies and regional medical centers, said Dr. Gina Mootrey of the CDC.

States also have not been told how to handle financial liability for vaccination side effects. That burden is usually handled by the federal government; so far this year, the National Vaccine Injury Compensation Program has paid almost $62 million in claims, none of them for smallpox vaccination.

Given the many concerns, panel members urged that any vaccination campaign be rolled out slowly, either state by state or in small steps within states. Some were clearly troubled by the sense of launching an unstoppable process: Once vaccinations begin, authorities will have to track hospital staff changes, vaccinate new hires and possibly give boosters every few years.