The atmosphere at the Royal Free hospital press conference in February 1998 was supercharged. As Andrew Wakefield and colleagues ex plained their Lancet paper on the 12 children whose parents believed became autistic after having the MMR jab, Arie Zuckerman, dean of the school of medicine, watched grim-faced. He took the microphone more than once to warn journalists of the harm they could do if they inflated this research into a big scare story.

But it did not need inflating. Scientists from a respectable institution had a paper published in one of the world's leading scientific journals which raised the possibility of a link between the MMR vaccination, a new form of bowel disease and autism. The bare facts were enough to panic every parent of young children.

I was one of them. My second child was four and her MMR booster was due. I put it off. When I did take her along, I was told by the nurse that she would be having the diphtheria, tetanus, pertussis (whooping cough) booster at the same time. I said I would bring her back for the MMR another day.

At the time, I thought Dr Wakefield and his team might have stumbled on a terrible truth. Four years, many scientific papers and much controversy later, I think the evidence stacks up against them and in favour of the triple vaccine - although I am prepared to believe in the theoretical possibility that it could trigger problems in a small number of children.

But that has not been proven. Dr Wakefield's original group of autistic children was tiny. There were eight whose autism began, from their parents' observation, after the MMR injection and one who had had measles. So is the vaccination to blame, or measles virus itself?

The main plank of the thesis is that MMR can trigger a form of bowel disease, which can in turn affect development. The Royal Free team found measles virus in the gut, but most scientists who tried to replicate the experiment failed. This week a long-awaited paper by John O'Leary, a pathologist from Dublin, revealed that he had found fragments of measles virus in gut tissue from the Royal Free children, but whether it is the MMR strain and whether it got there before the onset of bowel disease and autism is unknown.

Many respected scientists in Britain and the US have looked at the Royal Free's case and dismissed it. They cannot all be in the pockets of vaccine manufacturers.

In the end, every parent has to make a risk assessment. It is the known chances of death or damage from measles against the unproven possibility that some children may become autistic. In a sceptical era, it is not easy to trust the mainstream voice of science, but on this occasion - and reserving the right to change my mind as new evidence appears - I do.