BETHESDA, Maryland, USA: Mercury in some vaccines given to babies may be a contributory cause of autism and heart disease, new research here claims.

The study, by Dr Mark Geier, a physician with a PhD in genetics, and David Geier, a graduate student at the National Institutes of Health, who are both consultants in genetics based in Bethesda, Maryland, found that children who had received vaccines containing a preservative called thimerosal, which is almost 50 per cent mercury, were more than twice as likely to develop autism than children who did not.

Mercury, which is known to be neurotoxic, is being phased out of vaccines in America and Europe after scientists in the United States found the amount in vaccines exceeded federal safety limits.

The new study, published in the Journal of American Physicians and Surgeons, supports unpublished research carried out by the American Centres for Disease Control (CDC) in 2000. This showed an increased relative risk of autism of 2.48 for children who had received 75 micrograms of mercury, the amount in the British schedule.

The incidence of neurodevelopmental disorders and heart disease following the administration of Diphtheria, Tetanus and Pertussis (DTP) vaccines containing mercury compared with non-mercury vaccines was based on nationwide data in the United States.

The researchers claim they have produced "the first epidemiological evidence showing a direct association between thimerosal-containing childhood vaccines and neurodevelopmental disorders."

Thimerosal is used in some vaccines to prevent bacterial infection. Mercury is not present in the triple measles, mumps and rubella vaccine (MMR).

In the United States, there has been an increase in the number of vaccines containing mercury given to children since the 1980s. There has also been a dramatic increase in the number of children diagnosed with autism and other neurodevelopmental disorders such as attention deficit disorder. Several researchers have claimed that the rise in autism is linked to mercury-containing vaccines.

The researchers suggest that only a small proportion of susceptible children are at risk and that a combination of factors may be involved, including a possible genetic predisposition. They also support the controversial theory that the triple MMR jab may be the final trigger for autism. The mercury hypothesis is that a small sub-set of children have difficulty excreting mercury from the body. An accumulation of mercury can, in itself, cause brain damage and also lower the immune system so that the body cannot cope with the live viruses in the MMR.

In Britain, where there has also been a large increase in the number of children diagnosed with autism, the only mercury-containing vaccine given routinely to all children is the DTP vaccine. Some susceptible babies also receive flu jabs and a Hepatitis B vaccine which also contain mercury.

In Britain in 1990, the age at which the DTP is given was lowered from 3, 6 and 9 months to 2, 3 and 4 months and Dr Geier believes that this could be significant, as a smaller baby's immune system and brain are still developing and less able to cope with the toxic effects of mercury.

He said: "It is my opinion that our findings are definitely applicable to the UK."

Britain still uses the mercury-containing DTP, because the Department of Health says it is more effective. Vaccine manufacturerers are developing new DTP and flu jabs without mercury.

The UK Committee on the Safety of Medicines said that recent studies "showed no evidence of adverse developmental effects caused by thimerosal in vaccines according to the routine UK immunisation schedule. The balance of benefits and risks of thiomersal-containing vaccines therefore remains overwhelmingly positive."