Thiomersal and vaccines

Concerns about thiomersal and vaccines are commonly expressed by anti-vaccine activists. Claims relating to the safety of thiomersal, a mercury-based preservative used in vaccines, are refuted, but still subject to fearmongering, notably claims it could cause neurological disorders such as autism, leading to its removal from most vaccines in the US childhood schedule.^[1] This had no effect on the rates of diagnosis of pervasive developmental defects, including autism.^[2] Extensive scientific research shows no credible evidence linking thiomersal to such conditions.

Thiomersal (or thimerosal) is a mercury compound which is used as a preservative in some vaccines. Anti-vaccination activists promoting the incorrect claim that vaccination causes autism have asserted that the mercury in thiomersal is the cause.^[3] There is no scientific evidence to support this claim.^[4] The idea that thiomersal in vaccines might have detrimental effects originated with anti-vaccination activists^[5] and was sustained by them and especially through the action of plaintiffs' lawyers.^[6]

The potential impact of thiomersal on autism has been investigated extensively. Multiple lines of scientific evidence have shown that thiomersal does not cause autism. For example, the clinical symptoms of mercury poisoning differ significantly from those of autism.^[7] In addition, multiple population studies have found no association between thiomersal and autism, and rates of autism have continued to increase despite removal of thiomersal from vaccines.^[8] Thus, major scientific and medical bodies such as the Institute of Medicine^[9] and World Health Organization^[10] (WHO) as well as governmental agencies such as the Food and Drug Administration^[11] (FDA) and the Centers for Disease Control and Prevention (CDC)^[12] reject any role for thiomersal in autism or other neurodevelopmental disorders. In spite of the consensus of the scientific community, some parents and advocacy groups continue to contend that thiomersal is linked to autism^[13] and the claim is still stated as if it were fact in anti-vaccination propaganda, notably that of Robert F. Kennedy Jr., through his group Children's Health Defense.^[14][15] Thiomersal is no longer used in most children's vaccines in the United States, with the exception of some types of flu shots.^[16] While exposure to mercury may result in damage to brain, kidneys, and developing fetus,^[17] the scientific consensus is that thiomersal has no such effects.^[8][9]

This controversy has caused harm due to parents attempting to treat their autistic children with unproven and possibly dangerous treatments, discouraging parents from vaccinating their children due to fears about thiomersal toxicity^[18] and diverting resources away from research into more promising areas for the cause of autism.^[19] Thousands of lawsuits have been filed in the U.S. to seek damages from alleged toxicity from vaccines, including those purportedly caused by thiomersal.^[13] US courts have ruled against multiple representative test cases involving thiomersal.^[20] A 2011 journal article described the vaccine-autism connection as "perhaps, the most damaging medical hoax of the last 100 years".^[21]