A single dose of swine flu vaccine produces a robust immune response in pregnant women, one of the groups at high risk of dying from (A)H1N1 influenza, but young children need two shots, US clinical trials have shown.

"The immune responses seen in healthy pregnant women are comparable to those seen in healthy adults at the same time point after a single vaccination, and the vaccine has been well tolerated," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a statement.

No safety issues were reported during the trials, which began on September 9 and tested 120 women, all in the second or third trimester of pregnancy.

Twenty-one days after they were given the swine flu vaccine, 92 percent of pregnant women who received a single 15-microgram dose and 96 percent of expectant mothers who were given one, 30-microgram dose showed a robust immune response, showed the initial results of the trials, which are still ongoing.

The findings of the trials back up recommendations made last week by the World Health Organization (WHO), that "adults -- including pregnant women -- and adolescents, beginning at 10 years old" be given a single swine flu shot.

Separate tests conducted in the United States have shown that children aged six months to 35 months and three to nine years should have two doses of the H1N1 vaccination, Fauci told reporters Monday.