Scientists are to investigate how vaccinations given to pregnant women might affect the health of their unborn child, after research suggested that babies’ immune systems develop much earlier than thought.

A study published in the US Journal of Clinical Investigation yesterday found that the children of mothers who were given vaccinations against influenza started producing immune cells to combat the illness while still in the womb.

It is unclear whether such early production of antibodies has adverse or positive effects on an infant’s health.

Some researchers have suggested that exposure to vaccines, pollens and other agents during pregnancy may increase a child’s chances of developing allergies later in life. Such a hypothesis has been cited as the reason for rising rates of asthma and related illnesses.

Vaccinating pregnant women against flu is currently considered safe and the Department of Health is considering whether to implement recommendations made in December, by the Joint Committee on Vaccines and Immunisation (JCVI), the official watchdog, that all pregnant women be given the jabs when elderly and vulnerable patients are vaccinated during the winter flu season.

A team of researchers from Columbia University, New York, studied 126 women who were given flu vaccinations, which are already recommended for all mothers-to-be in the United States.

Specific antibodies found in the umbilical cord of their babies suggest that proteins contained in the jabs passed from mother to foetus, and stimulated production of immune cells in the developing child.

Antibodies were found in approximately 40 per cent of the cord blood samples, suggesting that the infants’ immune systems were capable of responding to agents passed from mother to child.

Previously, babies were thought to derive antibodies to protect them against illness from their mothers, via the placenta, not developing their own immune responses until some weeks after birth.

“These results have important implications for determining when immune responses to environmental exposures begin,” Rachel Miller, the lead author of the paper, said. “More research now needs to be done on what the effect to the child in later life is.”

Professor Miller said: “It is possible that the early stimulation of a child’s immune system might lead to the child developing asthma, eczema or other illnesses, but it is also possible that the beneficial effect of the vaccine might be conferred from mother to child, and protect the baby in early life.”

Donald Peebles, a consultant from University College London, and spokesman for the Royal College of Obstetricians and Gynaecologists, said that it was known that viruses and other agents could pass from mother to a developing baby, but more research was needed to determine the potential health effects.

“This study shows that the foetus is a good deal more sophisticated in developing its own immune responses than previously thought,” he said.

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