WASHINGTON - It's one thing to offer a risky, possibly deadly, smallpox vaccine. It's another to urge people to take it.

Federal officials planning for smallpox vaccinations are still debating this key issue: Should they advise people to get the shot, or just make it available and let everyone decide for themselves?

No matter what, a massive education campaign will be needed once the shots are available to the public, said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention. She said that officials plan to deliver detailed information directly to doctors, who can advise their patients. They are also planning information aimed directly at the public.

But she and other top health officials also want to offer clear recommendations. They say that advice could change as they learn more about reactions to the vaccine and as the threat of a bioterror attack on the United States goes up or down, but the government ought to be clear.

"It is difficult for people to make a decision without specific guidance," Gerberding said.

But that's not easy. What happens when people are injured - or killed - by the vaccine? Does the government carry extra legal liability if federal officials have recommended it?

Smallpox was wiped out worldwide two decades ago, but experts fear that the highly contagious, often fatal disease could return in an attack by terrorists or a hostile nation. Routine vaccinations ended in this country in 1972, meaning nearly half of the population is totally unprotected from the virus. Health officials aren't sure whether those vaccinated years ago have any residual protection.

The vaccine itself is risky. Based on studies from the 1960s, experts estimate that 15 out of every 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 entire nation could lead to nearly 3,000 life-threatening complications and at least 170 deaths.

The White House is still considering who should be offered the vaccine and how quickly. Top federal health officials have recommended offering it in stages, starting with about 500,0000 people who work in hospital emergency rooms and on special response teams. Next would be about 10 million others, including all health care workers and emergency responders. Finally, it would be offered to the general public after the vaccine - which is now experimental - is licensed, probably by early 2004.

Some in the White House are pushing for faster vaccinations, inoculating the entire nation in a matter of weeks. Under that scenario, the government would launch a massive education campaign aimed at persuading people that its benefits outweigh the side effects, according to one senior White House official who favors rapid vaccinations.

But what to tell people is far from settled.

"The issue is not the size or scope of the communications effort. It's about finding the communications message," said Dr. Michael Osterholm, a bioterrorism expert who is advising top federal officials.

The more conservative position is to simply offer the shots, tell people about the risk and let them decide.

That's what officials did last December with the anthrax vaccine. They offered it without a recommendation either way to people who were exposed to the deadly spores and finishing up two months of antibiotics. Health authorities didn't know what risk those people still faced or whether the vaccine would help. The result was considerable confusion.

"We didn't know really what to say," said Dr. D.A. Henderson, a top bioterrorism adviser at the Department of Health and Human Services. "We were criticized right, left and center for putting it out in this way."

Facing the same decision for smallpox vaccine, public health experts say there's no question that people need guidance.

"For something complex like this, I'm not sure it's fair to an individual to say `You can do or you can't do it, and here's some papers for you to read,'" said Dr. Jeffrey Koplan of Emory University, who headed the Centers for Disease Control and Prevention during the anthrax crisis.

Absent a specific recommendation, better educated Americans might get more information than poorer, less educated people - and thus make different decisions, he said. "That makes me very uncomfortable," he said.

That's just what happened with the anthrax vaccine: A military doctor gave congressional staffers a special briefing, telling them he trusted the shot, and dozens of them got it. But postal workers in Washington got mixed messages and far less special attention, and most said no. Two of the five people who died with anthrax attacks were postal workers.

"They just sent boatloads of literature to your house, paperwork," said Patricia Johnson, president of the local postal workers' union. "The average person wasn't just going to trust that."