A single dose of swine flu vaccine produces a robust immune response in pregnant women, one of the groups at higher risk of dying from (A)H1N1 influenza, initial results from US clinical trials have shown.

- The immune responses seen in these 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 for Allegies 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 were given the single 15-microgram dose and 96 percent of expectant mothers who were given one, 30-microgram dose showed a robust immune response, initial results of the ongoing trials showed.

Since the outbreak of the new strain of (A)H1N1 flu emerged in April, at least 100 pregnant women have been hospitalized in intensive care units in the US and at least 28 expectant mothers have died of pandemic swine flu.

Pregnant women have been turned away from flu vaccination clinics organized by state and county health authorities, where vaccine supply has been far outstripped by demand.

Expectant mothers have to have the injectable form of the swine flu vaccine, which is made with killed (A)H1N1 virus, which cannot cause infection.

The vaccine tested on the women did not contain the preservative thimerosal or an immune-boosting substance known as an adjuvant.

The vaccine is also available in the form of a nasal spray, but it is made with live, although greatly weakened, virus and is not advised for pregnant women, children under the age of two and people with chronic health conditions such as asthma.