Under the change, only troops that will serve in the Persian Gulf for at least 30 days will receive the shot, said spokesman Kenneth Bacon.
Forces deployed in South Korea, the only others currently receiving the vaccine under a program already curtailed to preserve scarce supplies, no longer will be vaccinated.
"Given the fact that Saddam Hussein has used chemical weapons in the past against Iran and against the Kurdish minority in his own country, we assess the risk to be greater in Iraq than we do right now in the Korean Peninsula," Bacon said.
"This is in order to conserve supplies while continuing to protect the people going to the highest-threat area," he said.
The Defense Department's only vaccine supplier, Bioport of Lansing, Mich., has experienced delays in obtaining certification and starting up its new manufacturing plant, which may not be operational for another year.
The Pentagon at first wanted to vaccinate all service members but cut back in the summer to only those serving in the Persian Gulf area and in Korea. Usage is running at about 17,000 doses a month.
The supply has since fallen to about 60,000 doses, meaning there is enough to give 5,000 shots a month to Gulf troops for a year, Bacon said. He said officials hope Bioport will have approval and be producing more vaccine by the third or fourth quarter of next year, and the Pentagon is working to find an alternate supplier as well.
Bacon said the military will save some 12,000 doses a month by temporarily discontinuing the program in South Korea, where the United States has helped maintain an armistice with North Korea since the end of the 1950-53 Korean conflict.
A relatively small, but vocal, number of military members have refused to take the vaccine, saying they fear potentially damaging side effects. In February, a Congressional panel recommended the Pentagon stop giving the vaccine.
Bacon asserted once again that the program is safe.
"We continue to believe that that anthrax vaccine is the best protection against a biological threat that is 99 percent lethal. It is safe," Bacon said. "And we will continue our program at the current level in the Gulf, and then ramp it up as soon as we can."
Anthrax is a naturally occurring virus. Dry anthrax spores, when inhaled, can be deadly to humans.
By PAULINE JELINEK