By Will Dunham

WASHINGTON, Sept 30 (Reuters) - Giving fewer doses of the anthrax vaccine protects a person just as well from the bacteria that causes the disease, which could extend limited supplies of the shots, researchers said on Tuesday.

Because anthrax can be used as a biological weapon, members of the U.S. military are among those getting the vaccine made by Maryland-based Emergent BioSolutions <EBS.N>.

Conrad Quinn of the U.S. Centers for Disease Control and Prevention and colleagues studied 1,005 U.S. adults given the vaccine. Those who received three doses over six months mustered a comparable immune response to those given the usual four doses over the same time.

The findings are part of the largest and most comprehensive study of this vaccine ever done, Quinn said. The study is ongoing, and the researchers are now looking to see if four shots given over a period of 3-1/2 years protect as well as eight shots over the same time period.

"Depending on the data at the end of the study, we have the potential to halve the doses that a person needs to get," Quinn, whose findings appear in the Journal of the American Medical Association, said in a telephone interview.

"If you have a certain amount of vaccine and now you don't have to give all of that to complete one series, you have more vaccine," Quinn added.

The study also found that injecting the vaccine into the muscle of the upper arm rather than under the skin reduced side effects.

Anthrax is a sometimes fatal infection caused by Bacillus anthracis bacteria. Its spores can be used as a biological weapon and are most deadly when inhaled.

Attacks involving anthrax-laced letters in the United States in 2001 killed five people.

Emergent is the government's only supplier of anthrax vaccine. But it involves many injections to work, and some who have received the shots complain about side effects.

Emergent and another Maryland company PharmAthene Inc <PIP.A> said last week they received contracts from the U.S. government to develop a new anthrax vaccine. PharmAthene gets up to $83.9 million. Emergent gets $29.7 million.

Two years ago, the government ended an $877.5 million deal with VaxGen Inc <VXGN.PK> to provide millions of doses of anthrax vaccine because regulators refused to approve new tests of the experimental product. (Editing by Maggie Fox)

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