Washington ? President Bush is moving toward approving a plan to eventually offer the smallpox vaccine to all Americans, starting with health care workers most likely to come into contact with a contagious patient, administration officials said Thursday.

He has not, however, signed off on key details or a final plan.

Bush is closer to a decision on shots for the military. Officials said he is inclined to approve a blueprint for vaccinating some U.S. troops against the disease, which was eradicated two decades ago but could return in biowarfare.

Bush’s top bioterrorism aides agree the vaccine should eventually be offered to the general public, and officials said Thursday Bush was comfortable with that plan. At issue is how fast to move ahead.

A once-feared disease, smallpox historically killed 30 percent of its victims. The virus also could be a powerful weapon. It is highly contagious and has no known treatment.

Routine vaccinations in the United States ended in 1972, making the population highly vulnerable to an attack.

Under the proposal for the military, the first personnel to receive the vaccine would be “first responders” ” troops responsible for assisting in domestic disasters, such as a bioweapons attack. They include medical specialists.

As many as 500,000 troops might eventually be inoculated, senior defense officials have said.

Of the 1.4 million men and women in the active-duty military, less than half have ever received the smallpox vaccine.