A large review of vaccination and medical records has revealed a significant link between exposure to mercury in vaccines and the incidence of neurodevelopmental disorders such as autism.
A possible link between autism and mercury exposure from the preservative thimerosal in vaccines remains highly controversial.
Thimerosal has now been removed from most childhood vaccines yet autism rates appear to continue their rise. Some commentators have suggested this completely exonerates thimerosal (and even mercury in general) from any role in autism.
It is possible however that thimerosal is simply one of many environmental triggers that can initiate neurodevelopmental disorders (NDs) such as autism in the genetically susceptible.
To determine if mercury from thimerosal containing vaccinations has any association with NDs researchers from The School of Public Health and Health Services at George Washington University in the United States examined records from the Vaccine Safety Datalink (VSD). The VSD was established in 1990 by the US Centers for Disease Control and Prevention (CDC) as a database of vaccination histories, health outcomes, and subject characteristics.
The VSD holds information about over 500,000 children, their vaccination histories and their health from birth through age 6. The researchers who carried out the latest review used data on 278,624 born in the period 1990-1996 who had received their first oral polio vaccination by 3 months of age.
The researchers then used the data from the VSD to calculate the exposure to mercury from thimerosal-containing vaccines each child had had from vaccinations from birth to 7 months and also from birth to 13 months.
The data was also used to determine the prevalence rates of neurodevelopmental disorders amongst the sample of children, as well as control health outcomes. These could be disorders or diseases that are not neurodevelopmental in nature or the absence of significant disease i.e. healthy children.
The researchers then conducted a statistical analysis on these sets of information to determine if there was an association between the prevalence of health outcomes and levels of exposure to mercury in thimerosal-containing vaccinations.
What they found was that there was a consistent and significant association between exposure to mercury in vaccines and autism spectrum disorders, tics, attention deficit hyperactivity disorder (ADHD), and emotional disturbances. Essentially what this means is that exposure to mercury in thimerosal-containing vaccinations early in life significantly increases the chances of a child developing autism and/or related disorders by between 1.7 and 4.5 times depending on the dose received.
The analysis showed that no such association existed with any of the control health outcomes. It seems therefore that exposure to mercury from thimerosal-containing vaccinations is a specific risk factor for autism and neurodevelopmental disorders and does not significantly increase the risk for unrelated health problems.
Lead researcher Assistent Professor Heather Young said: These findings are important because they demonstrate that children in a birth cohort with higher average mercury exposure than other birth cohorts had rates of neurodevelopmental disorders that were on average 2 to 4 times higher than the rates in the birth cohorts with lower exposures.
She continued, Because of the strong ecological associations found in this study, it is extremely important that additional studies be conducted using this data source, including case-control and cohort studies which, would allow the linking of individual exposure to disease.
Young and her research team were cautious in their conclusions however, recommending that routine childhood vaccinations should carry on to protect children from dangerous infectious disease but that efforts should be made to remove mercury from all childhood vaccinations (a process that is already well advanced in the US and many other countries).
Source: Young HA Geier DA Geier MR (2008) Thimerosal exposure in infants and neurodevelopmental disorders: An assessment of computerized medical records in the Vaccine Safety Datalink Journal of Neuroscience Epub May 14