The vaccine against swine flu is completely safe for pregnant women and could also help to protect their unborn child, the government's chief adviser on immunisation said yesterday.

Pregnant women have been identified as being particularly vulnerable to complications from the virus, but a poll last month said that almost half of expectant mothers could refuse a vaccination over fears about its safety.

Professor David Salisbury, director of immunisation at the Department of Health, said that such worries were "theoretical, with no evidence".

"The evidence is cumulating that it is worthwhile vaccinating pregnant women and that there is a benefit both to them and to their infant," he told a seminar of health professionals at the Royal College of Obstetricians and Gynaecologists.

A government leaflet giving advice to pregnant women who are considering the vaccine has been drawn up, he added.

The government's chief medical officer, Sir Liam Donaldson, told the seminar that, in contrast, the risks of contracting the swine flu virus were very real: "We know that pregnant women are at a higher risk of miscarriage and they are at higher risk of stillbirth and they are at higher risk of serious illness themselves. The problem is that we haven't really got enough data to be able to put numbers on it. We can't say to somebody what their risk is."

Having a vaccine in the midst of a major outbreak was a unique opportunity, Donaldson said. "In all previous pandemics we've been chasing after the virus and it's been almost too late to give a vaccine. This is the first time ever that it has been possible to have a vaccine in place close to the main peak of the pandemic."