New thimerosal study finds no link to autism but questions remain

Researchers say there is no link between mercury containing preservative thimerosal used in children's vaccines and autism, but is this the right question to ask?

The new research published in The New England Journal of Medicine does not investigate the effects of thimerosal on the developing brain drirectly but instead looks for an association between early exposure to the preservative in vaccines and how children have developed at ages 7 to 10.

A seperate study being carried out by the CDC is looking for more direct links between thimerosal and autism but results aren't expected to be releaed until next year. The current research was also conducted by CDC researchers in association with several managed-care organizations.

Researchers assessed a total of 1,047 children with varying degrees of exposure to thimerosal. Exposure was assessed by taking a thorough medical history for each child for their first 7 months of life. Levels of exposure to the mercury containing preservative ranged from 0 to nearly 200 micrograms, which is the most any child would receive if they took all of the thimerosal-containing vaccines according to the standard vaccination schedule. The possibility that the children's mothers may have been exposed to thimerosal during pregnancy was also looked at.

All of the children in the study were then assessed using 42 neurological and psychological examinations. These included I.Q. tests and assessments of how well the children recalled a list of names and whether they could repeat the names backward. Researchers also looked at their manual dexterity and whether they suffered from they stuttering or had tics.

The results will no doubt be hard for frustrated parents and advocacy groups to stomach. After the data collected was subjected to a battery of almost 400 different statistical measures, the researchers found 19 different possible associations between thimerosal and various mental outcomes. What may confuse many is that around half of these associations actually suggested that exposure to thimerosal actually improved the children's performance in certain tests.

The only result that raised concern amongst the researchers was the finding of an association between thimerosal and tics amongst boys. Tics are involuntary movements or sounds and this is not the first study to have made this association. Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the CDC, confirmed that this finding will certainly be investigated further.

This study was the third since 2004 to investigate the thimerosal and autism connection. Previous studies have come to similar conclusions.

Although these results seem to rule out a role for thimerosal in causing autism, studies such as this won't convince many. Instead parents and autism groups want to see biological studies showing directly that thimerosal at the levels these children were exposed to does not harm living cells or the development of the brain.

The problem with studies such as the one published this week is that they take too narrow a view of the situation. For example, take 2 children in the study, one exposed to 50 micrograms of thimerosal from vaccines and the other exposed to 200 micrograms. Researchers could look at the results of the testing they did on these 2 children and if no differences are seen could conclude that the child who received 200 micrograms of thimerosal was at no more risk of health problems than the other child. This may be a correct conclusion for thimerosal alone but thimerosal is only one of many sources of mercury.

If we consider the reality of the entire mercury situation, the children will have been exposed to mercury through many other routes than just thimerosal. Mercury is present in the air we breathe and in the food we eat, especially in the form of fish such as tuna. Mercury is also present in most electrical appliances and can easily be spread throughout the home.

When we look at total mercury exposure it may be that children who were exposed to low amounts of thimerosal were exposed to large amounts of mercury in other forms thus making their total mercury exposure equal or greater than those children in the study exposed to 200 micrograms of thimerosal from vaccines. As such, although the study seems to rule out a link between thimerosal alone and autism it certainly does nothing to clear up the debate over whether mercury itself is linked to autism.

Many experts now recognise the probablity that it is combinations of exposures acting synergistically that are at the root of many chronic illnesses. So rather than one toxin such as thimerosal causing a condition it is the cumulative effects of thimerosal and other sources of mercury, as well as completely different toxins, that are the problem. Surely it is obvious that research now needs to be conducted along these lines? Groups such as The Autism Society of America (ASA) have launched campaigns in recent months aimed at increasing awareness of the role of environmental toxins, individually and as a whole, in the development of autism.