WASHINGTON -- Federal Health authorities said this week they would carefully watch women who get the anthrax vaccine because of fears it might cause birth defects.

They said the possibility the vaccine could be harmful to pregnant women is slight, but they are taking precautions just to be sure.

"Three studies have been done looking at this issue in the military," a spokesperson for the Centers for Disease Control and Prevention (CDC) in Atlanta said in a telephone interview.

"Two showed no association (between the vaccine and birth defects) and one showed an association on a preliminary analysis. We still consider that plausibility of the vaccine causing birth defects is low because it is a killed vaccine."

Vaccines can be made using a live microbe, a killed microbe or just pieces of the disease-causing virus or bacteria. The killed versions are considered very safe but they are not always fully effective.

The CDC spokeswoman said her agency looked at all three of the studies before it decided to offer anthrax vaccine to people who may have been exposed to the potentially deadly spores in the mail attacks of October.

The study by the Naval Health Research Center that suggests a possible link with birth defects has not been completely analyzed or reviewed, and a check of the data could show no such link, the researcher said.

Scientific study is almost always slow and methodical, which is why researchers usually do not like to release data before they have checked and re-checked it.

"All three of the studies were considered as part of the decision to offer the vaccine," the spokeswoman said.

She said pregnant women were not forbidden to get the vaccine but were advised to think about possible risks to their babies when they filled out consent forms required to get the vaccine.

She said the CDC is carefully watching all the people who got the vaccine.

"Everyone will be followed for two years, including women who were or who may have been pregnant at the time of vaccination," she said.

---- Cautious Use ----

William Winkenwerder, the assistant secretary of Defense for Health Affairs, said earlier this week he was repeating and strengthening current policy that calls for cautious use of vaccines in servicewomen of childbearing age, especially during pregnancy.

"Although these study results are preliminary and there are significant concerns about the database that require further investigation before any conclusions can be made, we are taking these steps to reaffirm our existing policies," Reuters quoted Winkenwerder as saying in a statement.

More than 10,000 people took antibiotics after the October anthrax attacks, which affected postal workers, some journalists and congressional staffers. The CDC said earlier this month that 130 had opted to take the vaccine.

In December, the Health and Human Services Department offered the anthrax vaccine, usually available only to the military and to laboratory workers, to people who felt they needed extra protections in case they had spores still lurking in their lungs.

More than 400,000 servicemen and women have been given the vaccine as part of the military's anthrax immunization program.