About half the military's supply of anthrax vaccine will be stockpiled for civilians in case of a bioterror attack, expanding protection once meant almost exclusively for U.S. troops, the government said Friday.

Vaccination of U.S. troops abroad remains the top priority, officials said at a joint announcement at the Pentagon by the departments of Defense and Health and Human Services.

"There is a domestic need for access to the vaccine," so roughly half the millions of doses expected to be available eventually will be set aside for civilians, said William Winkenwerder, assistant secretary of defense for health affairs.

The vaccine will not be available for civilians to buy. And civilians will get the shots only if they are exposed to the deadly anthrax bacteria, said William Raub, an HHS deputy director for public health preparedness.

In contrast, the Pentagon program vaccinates soldiers ahead of time as a precaution against possible exposure.

Exposed citizens — whether police, emergency workers, or anyone — would get the vaccination along with antibiotics, a regime offered to postal workers and others exposed in last fall's still unsolved anthrax mail attacks.

Those, along with the Sept. 11 hijacked plane attacks on America, have prompted the new policy to split the drug between civilian and military communities, officials said.

During the anthrax attacks, the health department received 10,000 doses of vaccine from the Pentagon to start its stockpile. Only 100 people exposed to mailed anthrax received doses. Friday's announcement means millions of doses will be added to what remains.

The plan for the 50-50 split of the drug was revealed as part of an announcement on ramping up the Pentagon's troubled program.

It was started in 1998 to vaccinate all 2.4 million members of the active and reserve military, but it was radically reduced after factory violations by the nation's sole anthrax vaccine manufacturer left dwindling supply of the drug.

The Food and Drug Administration cleared Lansing, Mich.-based BioPort's manufacturing plant in January to produce the vaccine and release 500,000 doses already made.

After a three-month study considering the new domestic need, previous supply problems and other issues, the Pentagon decided to give the shots only to troops, essential civilians and contractors who are assigned for more than 15 days to "higher threat" areas of the world.

Winkenwerder declined to identify those areas or say how many troops would be given the vaccine. It is likely that these areas include the Persian Gulf and the Korean peninsula.

Believing Iraq and other nations had produced anthrax weapons, former Secretary of Defense William Cohen in 1997 ordered the armed forces immunized.

Shots started in 1998 for soldiers in areas of the highest risk — the Persian Gulf, then Korea — then moved beyond. As the drug shortage developed, the military scaled back, eliminating troops returning from deployments, then those in Korea and lastly those in the Gulf.

Since 1998, 2.1 million doses have been given to 525,000 people.

Announcement of the new policy not to vaccinate the entire force was delayed for nearly two months after the new Office of Homeland Security raised questions on how much vaccine might be needed by an estimated 2 million emergency workers who would be "first responders" to attacks in America.

Vaccinations for first responders are not planned but could be considered when supplies increase, officials said.

Homeland defense officials declined to comment.

For two years, the vaccine has been reserved for troops on special missions and for researchers. Starting immediately, the vaccine was to be given to a significantly greater number of troops, Winkenwerder said.

Increasing the troops vaccinated is the first step in rebuilding a program hobbled by both supply problems and refusal of a small percentage of soldiers to take shots they fear are unsafe. The government insists the vaccine is safe, and hundreds of people have been forced out of the military for refusing to take it.

The Pentagon is working on developing a new anthrax vaccine that would be faster to mass produce.

The Centers for Disease Control is studying whether sufficient protection against anthrax could be provided by giving the present vaccine in fewer doses.