Washington -- In deciding to offer the smallpox vaccine to the public on a voluntary basis, President George W. Bush is asking Americans to balance the known risks of side effects from the vaccine with the uncertain threat of a smallpox attack, specialists said Thursday.

There is a certain "trust me factor” in Bush's decision, said Ruth Faden, a bioethicist at Johns Hopkins University in Baltimore who has written about issues surrounding smallpox vaccination. "It's not possible for each of us to make a judgment about how likely it is that we would be attacked by smallpox weapons,” Faden said. "It is impossible for health professionals to make those judgments.”

It is the health officials who now have to gear up for a public education program to inform Americans of the risks and benefits of the vaccine and to alert millions who should not be vaccinated -- in the absence of a confirmed smallpox attack -- because they have health conditions putting them at higher risk for side effects.

Bush reportedly will unveil his plan Friday. He spoke this week of his decision to offer all Americans a chance to be vaccinated. "What's going to be very important is for us to make sure there's ample information for people to make a wise decision,” Bush said in a ABC-TV interview Wednesday.

The initial stage of the plan is expected to call for vaccination soon of about 500,000 military personnel and about 500,000 civilian health care workers and emergency personnel. The vaccine is not expected to be offered to the public until at least 2004, when enough newly manufactured product should be available.

Some health professionals had recommended against making vaccination widely available, given the potential for serious side effects and even death in rare cases. The Advisory Committee on Immunization Practices, which offers guidance to the federal government, had recommended in June against vaccinating the general population.

Dr. John Modlin, a Dartmouth Medical School pediatrician and chairman of the advisory committee, said Thursday he was not surprised by Bush's decision, which reflected advice the president received from administration health officials as well as outside groups. In October, top administration health officials, including Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention, said that they favored offering smallpox vaccine to the public.

"I think the president's decision is a political decision,” Modlin said, "but it is important to point out that it was made with the view that the president has of global events.” He said that he still regards the threat of a smallpox attack to be low and would not opt for a vaccination himself.

Michael Powers, an associate at the nonprofit Chemical and Biological Arms Control Institute in Washington, said that "there are lingering suspicions of whether Iraq has maintained samples of the smallpox virus” but little hard evidence that it has tried to incorporate it into a weapon. He said Bush's vaccination plan "represents a sort of hedge against what seems to be an unlikely threat.”

While the national security threat assessment may evolve over time, health specialists said the risk assessment for the vaccination also could change. "You think you know what the risk of the vaccine is,” said Dr. Samuel Katz, a Duke University pediatrician who has served on a federal advisory group on smallpox. "But all the data [about risks] were in the 1960s.” He said Friday there is a large group of Americans, perhaps 20 to 30 percent of the population, at risk of complications, including those whose immune systems are compromised by diseases such as AIDS or those with atopic dermatitis or who are taking drugs that suppress the immune system.

"Personally, I'm a bit dubious about the idea of making it available for the whole general public,” Katz said. Modlin said that there undoubtedly will be some adverse side effects among the first groups to be vaccinated. He suspects that publicity about those complications "will make people think twice about becoming immunized.”

The Centers for Disease Control and Prevention Thursday approved the pre-event vaccination plan New York State submitted last week.

State health officials are expecting a minimum of 100 health-care volunteers per each of the state's acute care hospitals. The federal government owns all stocks of the smallpox vaccine and will determine when doses can be distributed to states for vaccination of the volunteers.

Meanwhile, a poll released this week by the Robert Wood Johnson Foundation found that Americans are more willing than they were earlier this year to undergo smallpox vaccination.

Staff writer Delthia Ricks contributed to this story.