Giving fewer doses of the anthrax vaccine could be just as effective, offering protection from the bacteria that causes the disease, a new study has found. Researchers at the U.S. Centers for Disease Control and Prevention studied 1,005 U.S. adults given the vaccine. Participants were divided into five groups that got either three or four doses of the vaccine under the skin or into the muscle of the upper arm. They were followed over six weeks.

People who received three shots of the anthrax vaccine in an arm muscle showed the same immune response as those who got four standard injections under the skin over the same time.

The difference was that those who received the vaccine into the muscle of the upper arm experienced fewer side effects that those who got standard injection into the skin.

Conrad Quinn of the U.S. Centers for Disease Control and Prevention, lead-author of the study, said the findings are part of the largest and most comprehensive study of the anthrax vaccine ever done. The study will be published in the Journal of the American Medical Association.

The CDC’s Advisory Committee on Immunization Practice is planning to revise its recommendations on use of BioThrax vaccine now.

Anthrax is an acute disease in humans and animals caused by the bacterium Bacillus anthracis which is highly lethal in some forms. Its spores can be grown in vitro and be used as a biological weapon. When spores are inhaled, or come into contact with a skin lesion on a host they reactivate and multiply rapidly.

People get infected with the bacterial disease through inhaling, ingesting or touching the spores. Its fatality rate is of 45 percent to 90 percent. Five people died in US in 2001, from anthrax poisoning.

Human and animal vaccines are based on live-attenuated strains, anthrax spores, or cell-free vaccines prepared from bacterial supernatants or protein subunits, usually adsorbed onto aluminum salts, such as aluminium hydroxide. An FDA-licensed vaccine, produced from one non-virulent strain of the anthrax bacterium, is manufactured by BioPort Corporation, subsidiary of Emergent BioSolutions. The vaccine, named BioThrax, is commonly called Anthrax Vaccine Adsorbed (AVA). The vaccine is usually administrated in a six-dose primary series at 0,2,4 weeks and 6,12,18 months; annual booster injections are required thereafter to maintain immunity.

There has been a dispute about the anthrax vaccine, which started years ago, when a federal judge suspended the vaccination program in 2004, finding errors in FDA’s process for approving the drug. After FDA redid the process and declared it safe, the anthrax vaccinations have been reintroduced in the military system. Because anthrax can be used as a biological weapon, members of the U.S. military are among those getting the anthrax vaccine.

The anthrax attacks in the United States in 2001 killed five people and sickened seventeen others. The deadly anthrax mailings were sent to media organizations and politicians less than a month after the September 11, 2001, suicide attacks that killed thousands of people in New York, Washington and Pennsylvania.