ATLANTA (Reuters) - Studies of the health effects of the anthrax vaccine given to U.S. military personnel have found no unexpected adverse reactions, federal health officials said on Thursday.

The Centers for Disease Control and Prevention (CDC) said a small percentage of service members complained of muscle or joint aches, headache, or fatigue after being vaccinated.

The U.S. Department of Defense, concerned about the threat of biological warfare, launched a program in May 1998 to administer a six-dose anthrax vaccine to all 2.4 million active and reserve military personnel by the year 2004.

As of February, more than 400,000 service members had received the mandatory anthrax inoculations. About 350 military personnel have refused vaccination, in some cases because they were concerned that the vaccine was unsafe or ineffective.

The CDC said three separate surveys found no unexpected reactions to the vaccine. The studies indicated that women were more likely than men to report minor side effects, but researchers said they did not know why.

In Korea, 1.9 percent of 4,348 service members who had received at least two doses of the vaccine reported minor side effects that limited their work performance, but the effects generally only lasted a day and did not require medical intervention.

A study by the Tripler Army Medical Center, in Honolulu, Hawaii, found that 7.9 percent of 595 health-care workers surveyed sought medical advice or took time off duty because of minor side effects after receiving their first dose of vaccine.

Overall, the Department of Defense said it has received a total of 428 reports of adverse reactions. In only 36 of those cases did military personnel require hospitalization or more than one day off duty to recover from the side effects.

The CDC said early research indicates that injecting the vaccine into muscle, instead of directly below the skin, can reduce short-term side effects.

Anthrax spores are odorless, invisible and highly lethal when inhaled. The Department of Defense describes the bacteria as 100,000 times more potent than the deadliest chemical-warfare agent.

Defense officials maintain that sending out unvaccinated troops could cause the death of large numbers of soldiers, sailors, airmen or Marines.

The General Accounting Office said last week that the U.S. military is facing a shortage of the anthrax vaccine and may be forced to suspend the program in July unless new supplies become available.