The MMR (measles, mumps, rubella) and DTP (diphtheria, tetanus, pertussis) vaccines increase the rate of seizure by more than three times, the most definitive research into the two jabs has discovered.

These findings may well have wide ranging implications, even affecting government vaccination plans, because they have been prepared by the authoritative and influential US Centers for Disease Control and Prevention in Atlanta, Georgia, and because they are based on the most complete data yet compiled.

Using database technology, the CDC pulled together virtually every piece of research and data into adverse reactions to the two vaccines. In all, they were able to monitor the progress of 500,000 children, by far the most ever observed by any single piece of research.

They identified 34 major side effects to the jabs, ranging from asthma, blood disorders, infectious diseases, diabetes, and neurological disorders, including meningitis, polio and hearing loss.

But it was the incidence of seizure that leaped off the graph. The rate of seizure increased three times above the norm within the first day of a child receiving the DTP shot, and the rate rose 2.7 times within four to seven days of a child being given the MMR shot, increasing to 3.3 times within eight to 14 days.

The effects of the DTP shot were immediate, causing seizures to increase three times the normal within 24 hours of the jab being given, but then falling off rapidly to just 0.6 times the level of seizure after the first day. The MMR vaccine, however, had a far slower effect, only reaching its most dangerous period within eight days to two weeks after the jab was administered.

The seizures were often serious, the CDC reports, with a quarter of all cases being treated in hospital.

The CDC is carrying out further research into findings before submitting a final paper, although it may be too late to halt President Clinton's plans to have all children under the age of 2 vaccinated, and to cause any major rethink in Britain where the government is funding a £20m campaign to have all children vaccinated against an expected measles epidemic next year.