Since 1999 there has been a controversy about whether thimerosal containing vaccines contribute to the causes of Autism Spectrum Disorders (ASDs) and neurodevelopmental disorders (NDDs) in children. Thimerosal is a mercury-based preservative used to manufacture multidose vaccines as it helps to prevent the growth of bacteria and fungi and so can protect the immunized from infections at the site of the immunization. One of the reasons for the concern was that thimerosal contains ethylmercury and not methylmercury.
But since there were no guidelines for the safe amount of ethylmercury in the human body, the guidelines for methylmercury were used on the false assumption that the body handles them both in the same way. In trying to settle the controversy two main issues were brought up. One is the connection between the chemistry of thimerosal and a direct link with autism and the other is the removal of thimerosal from vaccines and the effect on the level of incidences of autism.
The Public Health Agency of Canada does not believe that thimerosal causes autism or other neurodevelopmental disorders. In the paper, Thimerosal updated statement ( Law and Primeau , 2007) the authors reported two reviews of the evidence by the Immunization Safety Review Committee of the US Institute of Medicine (IOM) in 2001 and 2004 . In 2001 they concluded that the evidence is inadequate to accept or reject a causal relationship between thimerosal exposures from childhood vaccines and neurodevelopmental disorders.
They also believed that the hypothesis that thimerosal exposure through the recommended childhood immunization schedule [in the US] has caused neurodevelopmental disorders is not supported by clinical or experimental evidence. The subsequent review in 2004, which included studies from Denmark, Great Britain, Sweden and the US concluded that the evidence favors rejection of a causal relationship between thimerosal containing vaccines and autism. The paper also reported Canadian data from a study in Quebec of 188 children to support the lack of association between thimerosal exposure and pervasive developmental disorders (PDDs).
Despite the belief that thimerosal exposure has not been scientifically proven to be linked to autism, the Public Health Agency states that in large concentrations over extended periods of exposure, mercury can cause damage to brain and kidney.
In a critical review of the published data Parker et al (2008) analyzed twelve published studies from Denmark, Sweden, UK and US. In an attempt to address the debate that continued about the possible link between thimerosal and autism. The authors first state that in 2001 thimerosal in quantities sufficient to act as a preservative was removed form all vaccines in the childhood immunization schedule in the US except in some influenza vaccines. They state that although trace amounts are present in some vaccines, the amounts are so small that exposure is inconsequential.
The Immunization Safety Review Committee of the Institute of Medicine concluded in 2001 that the evidence is insufficient to accept or reject a causal relationship between exposure to thimerosal and NDDs. In order to examine this claim more fully the authors analyzed the twelve studies.
They concluded that the four studies supporting an association between thimerosal exposure and NDDs including autism were by the same authors and used overlapping data sets and contained critical methodological flaws that render the data and their interpretation non contributory. On the other hand they found that the studies that did not report an association were well designed and appropriately analyzed. They also make the point that data from Denmark and Sweden where exposure to thimerosal in vaccines was eliminated in 1992 showed that rates of autism continued to increase. Thus questioning thimerosal as a possible contributing cause to autism.
Nelson and Bauman (2003) examined the claim that there is a link between thimerosal and autism from three different angles. They first compared clinical manifestations of autism and mercury toxicity. They found no commonality in motor manifestations. Other signs such as hypertension, skin eruptions evident in mercury toxicity were not present in autism.
They concluded that the typical clinical signs of mecurism are not similar to the typical clinical signs of autism. The second angle was to look at the onset of autism after immunizations. According to them the onset is very difficult to establish and so they cannot prove or disprove a role of the vaccines. Thirdly they questioned the existence of mercury levels in autistic children. They found no evidence to show significant amounts of mercury in the hair, urine, or blood of autistic children. They also point out that no evidence has been provided to show that chelation therapy led to the improvement in children with autism.
Another point raised by these authors is the question of whether reducing levels of mercury can affect the levels of incidence of autism. They referred to a study done in Japan during and after a toxic outbreak in the 1950s and the 1960s. The results showed on the contrary, higher rates of autism in children born between 1966 and 1974 than in the periods just after the outbreaks.
Though the controversy may not be over, and some will still not feel safe enough to vaccinate their children, the research has not been able to establish a reliable link between thimerosal in vaccines and the occurrence of autism.
Law, B., & Primeau, M. (2007). Thimerosal:updated statement. Canada Communicable Disease Report. Vol.33. Retrieved August 25, 2008. http://www.phac-aspc.gc.ca/publicat/ccdr-rmtc/07vol33/acs-06/index_e.html
Nelson, K.B., & Bauman, M.L. (2003). Thimerosal and autism. Pediatrics Vol. 111, Iss.3, pp.674-679.
Parker, S., Schwartz, B., Todd, J., & Pickering, L. (2004). Thimerosal-containing vaccines and autistic spectrum disorder: a critical review of the published original data. Pediatrics. Vol.114, Iss.3, pp.793-804