Bush will announce the program Friday and shots are expected to begin in January, senior administration officials said Wednesday.

The shots will be mandatory for about 500,000 military personnel and recommended for another half-million who work in hospital emergency rooms and on special smallpox response teams.

The general public will be offered the vaccine on a voluntary basis as soon as large stockpiles are licensed, probably early in 2004, though the government will not encourage people to get them.

In making the decision, Bush had to weigh the risks of the often-deadly disease against the dangers of the vaccine, which produces more serious side effects than any other vaccine dispensed in this country.

In his limited public comments on the matter, Bush has emphasized his concerns about the vaccine, and he said Wednesday that people will have to consider its dangers.

"It's going to be very important for us to make sure there's ample information for people to make a wise decision," Bush said in an interview broadcast Wednesday on ABC-TV's "World News Tonight" program.

Federal health officials are set to do just that, preparing a massive public education campaign about both the disease and the vaccine. Polls, including one released Wednesday, show most people would choose to receive the vaccine if given the chance. But health officials fear that many do not adequately understand the risks.

Based on studies from the 1960s, about 15 out of every 1 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.

"The success of a vaccination program is going to depend on our success in communicating with people accurately and openly," Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention, said Wednesday.

Smallpox, once among the most feared diseases on Earth, killed hundreds of millions of people in past centuries, but it hasn't been seen in this country since 1949 and was declared eradicated worldwide in 1980. But experts fear that it could be used by hostile nations or terrorist groups in an attack. Intelligence experts believe that four nations, including Iraq, have unauthorized stocks of the virus.

Vaccinating people against the disease makes enemies less likely to use it, said Sen. Bill Frist, R-Tenn., who has been encouraging Bush to offer the vaccine widely.

"It's the right step to protect the American people and its the right step to make our nation less vulnerable to those who would use smallpox to terrorize our citizens," he said. "This is a difficult decision, but it is the right decision."

Bush's vaccination plan represents a remarkable journey of public policy. Just this summer, federal health advisers were recommending a much more conservative vaccination program, perhaps 15,000 to 20,000 people total.

Many, including Dr. D.A. Henderson, a federal bioterrorism adviser who led the global campaign to wipe out the disease, believed this was enough. If smallpox appeared again, he and others reasoned, people could be vaccinated in an emergency campaign. The vaccine is effective if delivered a few days after being exposed to the disease.

He said Wednesday that he hopes the administration will evaluate the results of early vaccinations before actually offering the shot to the public.

Despite concerns, the administration's vaccination plan scaled up quickly as the government bought enough vaccine to cover everyone in the nation and as the nation moved closer to war with Iraq.

For the civilian population, the plan closely tracks recommendations from the administration's top health officials, rejecting the more aggressive suggestions by some, including Vice President Dick Cheney, that all Americans be offered the vaccine right away.

Under the plan, the vaccine will be offered first to people most likely to encounter a highly contagious smallpox patient. That includes people who work in hospital emergency rooms, where sick patients might come for help, and those on special state teams that would investigate suspicious smallpox cases.

States submitted plans this week explaining how many people they plan to inoculate during this first stage. With all but five state plans in, it looks like there will be close to a half-million people included, Gerberding said.

In a second stage, the shot would be offered to all other health care workers plus emergency responders such as police, fire and emergency medical technicians. That is likely to total roughly 10 million people.

Eventually, the vaccine will be made available to all Americans, though the government will probably not encourage them to get it, according to senior administration officials.

Unlike civilians, shots would be compulsory for the military - a sensitive proposition given that some troops rebelled at mandatory anthrax vaccines, which are much less dangerous. It wasn't clear whether all military personnel being deployed to the Persian Gulf region for possible war with Iraq would receive the shots.

In an interview with CNN in Qatar, Defense Secretary Donald H. Rumsfeld said he had signed off on a plan for giving the vaccine to a limited number of troops, starting with those who are designated to respond to domestic emergencies and troops who would be earmarked for deployment to conflict in the Middle East.

Routine smallpox vaccinations ended in the United States in 1972, meaning nearly half the population is without any protection from the virus. Health officials aren't sure whether those vaccinated decades ago are still protected from the disease.

Once vaccinations begin, it will be important that certain people not get the vaccine because they face particular risk of side effects. That includes cancer patients, organ transplant recipients, people with HIV, pregnant women and people with a history of eczema.

People who live with others who have these conditions also should not be vaccinated, because the live virus used in the vaccine can sometimes escape the inoculation site and infect others.

It's these side effects that concern people like Henderson.

"I must confess, I thought we'd seen the very last of smallpox vaccine," he said Wednesday. "It troubles me every time I think about this."

Associated Press Writer Scott Lindlaw contributed to this report.