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No thiomersal link to autism but speech delay doubts remain

A US Government study has provided further reassurance that there is no link between infant vaccines containing the mercury-based preservative thiomersal and autism or attention deficit disorder.

But researchers from the US centers for disease control and prevention (CDC) called for further research into associations they found between cumulative thiomersal exposure and speech and language delays and tics.

The two-phased retrospective study published in Pediatrics (November) looked at 140,000 babies born at three separate centres between 1992 and 1999 and monitored them at one, three and seven months of life when they were given thiomersal-containing vaccines (TCVs). Research showed 'no consistent significant associations were found between TCVs and neurodevelopmental outcomes'.

This supports recent advice from the Committee on Safety of Medicines which said there was no link between the vaccines and neurodevelopmental disorders. It also found ethylmercury in vaccines was rapidly excreted by infants.

The committee's statement was prompted by controversial research by US team Geier and Geier published earlier this year that claimed 90 per cent of neurodevelopmental disorders are vaccine-linked.

The new study by the CDC measured mercury exposure from TCVs following the

recommended immunisation schedule in the first seven months, which included DTP, Hib and hepatitis B vaccine. The research measured associations with a range of neurodevelopmental outcomes, including autism, ADD, speech and language disorders and tics.

Researchers found no significant associations with either autism or ADD. The first stages of the study found associations between cumulative thiomersal exposure and speech and language delays, and tics. But these were not picked up as significant in the second phase of the study, where researchers attempted to confirm the positive findings in a third centre.

Researchers said this may be due to a lack of consistency between the two phases of the study. 'We believe additional investigation is required because of the widespread exposure from vaccinating virtually the entire birth cohort of the US and the importance of speech and language disorders among adolescents,' they added.

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