The apparent soaring rate of autism in recent years is almost certainly not due to the MMR injection, a major study suggests.
One of the strongest arguments linking the jab to autism has been the fact that the number of diagnoses has spiralled upwards since the late 1980s - around the time that the combined measles, mumps and rubella vaccine was introduced.
However, a major statistical analysis of UK children, published on the website of the British Medical Journal, has found that there appears to be no clear link between MMR and the rise.
Put simply, the number of cases has continued to rise even though MMR coverage has remained roughly the same.
If MMR was the cause of illness, say the experts, the number of autism diagnoses would also have levelled off by now.
Dr Hershel Jick, an associate Professor of Medicine from Boston University School of Medicine, who helped author the research, said: "The correlation does not indicate that MMR is causing autism. We have no idea know what's causing it, but it isn't MMR."
He added that he hoped the research would provide extra reassurance for parents.
The study looked at groups of boys born between 1988 and 1993. The MMR jab was introduced in 1988.
Since that time, the incidence of newly-diagnosed autism has increased sevenfold, but this has been a steady rise over that period, not simply a marked jump over the first few years.
The study reports: "When the incidence of an illness is rising rapidly in each birth year cohort at the same time that an exposure (to a possible cause) is steady and almost universal, the exposure cannot be the explanation for the rapid increase in incidence."
Dr Jick suggested that the increased awareness of autism as a diagnoseable disease during this period meant that many cases which previously would have been missed or misdiagnosed are being identified.
The other option is an environmental factor which has yet to be spotted.
Whether the study will allay genuine fears among parents is yet to be seen, but this is one of the strongest pieces of evidence yet that MMR may be blameless.
Immunization rates, particularly in the inner cities, have fallen sharply since the MMR scare, prompting fears of the return of measles outbreaks, which could cause permanent damage or even death in a small number of cases.
There is still no firm statistical evidence of a link, and the UK Department of Health is preparing to spend £3m on a publicity campaign to reassure parents.
There is some biological evidence which seems to link the presence of measles virus in the gut with autism, and some scientists have suspected that the delivery of more than one vaccine virus at once may be creating a risk.
Campaign groups want the department to allow the wide circulation of single vaccines against measles, mumps and rubella, which are believed by some to be safer.