Thimerosal, a compound that contains ethyl mercury, has been used as an additive to biological therapies and vaccines because of its effect in preventing bacterial contamination, particularly in opened, multidose vials. In 1997, the United States Food and Drug Administration (FDA) Modernization Act called for a review and assessment of the risk of all mercury-containing foods and drugs. This action stimulated the United States Public Health Service and the American Academy of Pediatrics to issue a joint statement in 1999 (18
) calling for the removal of thimerosal from vaccines. This action was undertaken as a precautionary measure; there was no evidence that ethyl mercury was harmful at the doses being administered to infants.
Of note, at that time in Canada, in contrast to the United States, the regularly used infant immunization product (pentavalent DTaPIPVHib vaccine) did not contain thimerosal. Only two infant thimerosal-containing vaccines were used – hepatitis B vaccine and influenza vaccine; the latter was not administered to infants younger than six months of age, the age/size of infant of concern. Hence, any concerns about excessive ethyl mercury exposure in young Canadian infants were without foundation. Since 1999, several studies (19
) have been conducted to evaluate the safety of thimerosal in vaccines. These studies were reviewed in detail by the IOM (10
) in 2001 and 2004 with a focus on autism. The IOM Committee concluded that the evidence favoured rejection of a causal relationship between thimerosal-containing vaccines and autism, as well as MMR vaccine and autism (10
). In the absence of experimental or human evidence that vaccination affects metabolic, developmental, immune, or other physiological or molecular mechanisms that are related causally to development of autism, the IOM concluded that the hypotheses generated to date are theoretical. In a separate critical review (24
) of published original data, a link between thimerosal-containing vaccines and ASD was not shown. Epidemiological studies that supported a link demonstrated significant design flaws that invalidated conclusions of these studies (10
). Additional data from Canada published since 2004 also showed no association between thimerosal-containing vaccines and autism (25
An important factor to consider is what has happened to autism rates since the removal of thimerosal from vaccines. In studies from Canada (25
), Denmark (20
) and the United States (26
) the rates of autism have continued to increase despite removal of thimerosal from vaccines.
Thus, the evidence is in, and the assessment of purported causality is clear. The MMR vaccine and immunization with thimerosal-containing vaccines are not causally associated with, nor are they a cause of, autism or ASD. There is mounting evidence (27
) that ASD has a strong genetic component – a very plausible cause for the disorder.