John Sullivan, professor of paediatrics at the University of Massachusetts, said that between 15 and 45 per cent of babies born to HIV-positive mothers will become infected themselves.
Professor Sullivan said he had found that at least half and possibly up to three-quarters of the infants pick up the Aids virus as they pass down the birth canal. The scientists hope to stop the babies becoming infected by adopting an approach that has already proved successful in preventing more than 95 per cent of transmissions of hepatitis B, which is also most likely to be passed on at the time of birth.
The babies in next month's US trial will be given genetically engineered therapeutic vaccines, in the hope that these will stimulate the infant's own immune response before the HIV-1 infection has a chance to take grip.
The engineered immunoglobulin drug was developed by two of the world's leading biotechnology companies. Fewer than 100 babies will take part in the initial trial. They will receive one shot on the first day of their lives, one at four weeks, and a third at eight weeks.
Not all of the babies will definitely have been infected with HIV-1. It is not possible to establish whether the infants are infected straight after birth, since the amount of virus picked up is often too small to detect. Scientists do not know exactly how infection takes place during birth, although Professor Sullivan pointed out that a baby's mucus membranes are more permeable than those in adults.
It is also possible that infants swallow viral material which their stomachs are insufficiently acidic to counteract.
Birth by Caesarean section might offer protection, but the mother's infected blood and mucus would still be a probem.
Another speaker, Dr William Haseltine, a human genome researcher from Maryland, was extremely pessimistic about recent research attempting to thwart the Aids virus, and on the prospects for finding successful vaccines.
'It is still premature to predict that we can have a cure for this disease,' he said. 'The state of science in general is not sufficiently advanced to allow us to predict that we can ever have a vaccine for Aids.'
Professor Sullivan insisted, however, that there were grounds for optimism. 'If we think about the vaccines that are effective, and that are in use today . . . sometimes we are just very lucky and get a vaccine that works, but we don't know exactly how it works.'
About 6,000 HIV-positive women give birth to 1,500 infected babies every year in the US. In Africa, about 150,000 HIV-1 infected babies are born each year.Reuse content