Healthy pregnant women need only one dose of H1N1 influenza vaccine to mount a robust immune response, U.S. health officials say.

Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, part of the U.S. Department of Health and Human Services, said a trial began Sept. 9 and reached its target enrollment of 120 volunteers in mid-October.

All participants are 18- to 39 years of age and began the study in their second or third trimester -- 14 to 34 weeks -- of pregnancy.

The trial participants were divided at random into two groups -- half received two doses of a 15-microgram vaccine and the other half received two doses of a 30-microgram vaccine. The two injections of vaccine were spaced three weeks apart.

In the ongoing clinical trial of 25 women who received a single 15-microgram dose of the vaccine, the H1N1 flu vaccine elicited an immune response likely to be protective in 92 percent, or 23 of 25, of these women, Fauci said.

In 25 women who received a single 30-microgram dose of the vaccine, the H1N1 flu vaccine elicited an immune response likely to be protective in 96 percent, or 24 of 25, of these women.

The vaccine was manufactured by Sanofi Pasteur in its plant in Swiftwater, Pa., and does not contain the preservative thimerosal or an immune boosting substance known as an adjuvant, Fauci said.