thiomersal and vaccines thiomersal and vaccines 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. there is no scientific evidence to support this claim. the idea that thiomersal in vaccines might have detrimental effects originated with anti-vaccination activists and was sustained by them and especially through the action of plaintiffs' lawyers. 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. 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. thus, major scientific and medical bodies such as the institute of medicine and world health organization (who) as well as governmental agencies such as the food and drug administration (fda) and the centers for disease control and prevention (cdc) 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 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. thiomersal is no longer used in most children's vaccines in the united states, with the exception of some types of flu shots. while exposure to mercury may result in damage to brain, kidneys, and developing fetus, the scientific consensus is that thiomersal has no such effects. 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 and diverting resources away from research into more promising areas for the cause of autism. thousands of lawsuits have been filed in the u.s. to seek damages from alleged toxicity from vaccines, including those purportedly caused by thiomersal. us courts have ruled against multiple representative test cases involving thiomersal. a 2011 journal article described the vaccine-autism connection as "perhaps, the most damaging medical hoax of the last 100 years". history thiomersal (also spelled thimerosal, especially in the united states) is an organomercury compound used as a preservative in vaccines to prevent bacterial and fungal contamination. following a mandated review of mercury-containing food and drugs in 1999, the centers for disease control and prevention (cdc) and the american academy of pediatrics (aap) determined that under the existing vaccination schedule "some children could be exposed to a cumulative level of mercury over the first 6 months of life that exceeds one of the federal guidelines on methyl mercury." they asked vaccine makers to remove thiomersal from vaccines as quickly as possible as a precautionary measure, and it was rapidly phased out of most us and eu vaccines, but is still used in multi-dose vials of flu vaccines in the u.s. no vaccines in the european union currently contain thiomersal as a preservative. in the context of perceived increased autism rates and increased number of vaccines in the childhood vaccination schedule, some parents believed the action to remove thiomersal was an indication that the preservative caused autism. it was introduced as a preservative in the 1930s to prevent the growth of infectious organisms such as bacteria and fungi, and has been in use in vaccines and other products such as immunoglobulin preparations and ophthalmic and nasal solutions. vaccine manufacturers have used preservatives to prevent microbial growth during the manufacturing process or when packaged as "multi-dose" products to allow for multiple punctures of the same vial to dispense multiple vaccinations with less fear of contamination. after the fda modernization act of 1997 mandated a review and risk assessment of all mercury-containing food and drugs, vaccine manufacturers responded to fda requests made in december 1998 and april 1999 to provide detailed information about the thiomersal content of their preparations. a review of the data showed that while the vaccine schedule for infants did not exceed fda, agency for toxic substances and disease registry (atsdr), or who guidelines on mercury exposure, it could have exceeded environmental protection agency (epa) standards for the first six months of life, depending on the vaccine formulation and the weight of the infant. the review also highlighted difficulty interpreting toxicity of the ethylmercury in thiomersal because guidelines for mercury toxicity were based primarily on studies of methylmercury, a different mercury compound with different toxicologic properties. multiple meetings were scheduled among various government officials and scientists from multiple agencies to discuss the appropriate response to this evidence. there was a wide range of opinions on the urgency and significance of the safety of thiomersal, with some toxicologists suggesting there was no clear evidence that thiomersal was harmful and other participants like neal halsey, director of the institute of vaccine safety at johns hopkins school of public health, strongly advocating removal of thiomersal from vaccines due to possible safety risks. in the process of forming the response to this information, the participants attempted to strike a balance between acknowledging possible harm from thiomersal and the risks involved if childhood vaccinations were delayed or stopped. upon conclusion of their review, the fda, in conjunction with the other members of the us public health service (usphs), the national institutes of health (nih), cdc and health resources and services administration (hrsa), in a joint statement with the aap in july 1999 concluded that there was "no evidence of harm caused by doses of thimerosal found in vaccines, except for local hypersensitivity reactions." despite the lack of convincing evidence of toxicity of thiomersal when used as a vaccine preservative, the usphs and aap determined that thiomersal should be removed from vaccines as a purely precautionary measure. this action was based on the precautionary principle, which assumes that there is no harm in exercising caution even if it later turns out to be unnecessary. the cdc and aap reasoned that despite the lack of evidence of significant harm in the use of thiomersal in vaccines, the removal of this preservative would increase the public confidence in the safety of vaccines. although thiomersal was largely removed from routine infant vaccines by summer 2001 in the u.s., some vaccines continue to contain non-trace amounts of thiomersal, mainly in multi-dose vaccines targeted against influenza, meningococcal disease and tetanus. in 2004 quackwatch posted an article saying that chelation therapy has been falsely promoted as effective against autism, and that practitioners falsified diagnoses of metal poisoning to "trick" parents into having their children undergo the process. as of 2008, between 2–8% of children with autism had undergone the therapy. rationale for concern reports of autism cases per 1,000 children grew dramatically in the u.s. from 1996 to 2007. it is unknown how much, if any, growth came from changes in autism's prevalence. although intended to increase public confidence in vaccinations, the decision to remove thiomersal instead led to some parents suspecting thiomersal as a cause of autism. this concern over a vaccine-autism link grew from a confluence of several underlying factors. first, methylmercury had for decades been the subject of widespread environmental and media concern after two highly publicized episodes of poisonings in the 1950s and 1960s in minamata bay, japan from industrial waste and in the 1970s in iraq from fungicide contamination of wheat. these incidents led to new research on methylmercury safety and culminated in the publication of an array of confusing recommendations by public health agencies in the 1990s warning against methylmercury exposure in adults and pregnant women, which ensured a continued high public awareness of mercury toxicity. second, the vaccine schedule for infants expanded in the 1990s to include more vaccines, some of which, including the hib vaccine, dtap vaccine and hepatitis b vaccine, could have contained thiomersal. third, the number of diagnoses of autism grew in the 1990s, leading parents of these children to search for an explanation for the apparent rise in diagnoses, including considering possible environmental factors. the dramatic increase in reported cases of autism during the 1990s and early 2000s is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness, and it is unknown whether autism's true prevalence increased during the period. nevertheless, some parents believed that there was a growing "autism epidemic" and connected these three factors to conclude that the increase in number of vaccines, and specifically the mercury in thiomersal in those vaccines, was causing a dramatic increase in the incidence of autism. advocates of a thiomersal-autism link also relied on indirect evidence from the scientific literature, including analogy with neurotoxic effects of other mercury compounds, the reported epidemiologic association between autism and vaccine use, and extrapolation from in vitro experiments and animal studies. studies conducted by mark geier and his son david geier have been the most frequently cited research by parents advocating a link between thiomersal and autism. this research by geier has received considerable criticism for methodological problems in his research, including not presenting methods and statistical analyses to others for verification, improperly analyzing data taken from vaccine adverse event reporting system, as well as either mislabelling or confusing fundamental statistical terms in his papers, leading to results that were "uninterpretable". publicity of concern several months after the recommendation to have thiomersal removed from vaccines was published, a speculative article was published in medical hypotheses, a non-peer-reviewed journal, by parents who launched the parental advocacy group safeminds to promote the theory that thiomersal caused autism. the controversy began to gain legitimacy in the eyes of the public and gained widening support within certain elements in the autism advocacy community as well as in the political arena, with u.s. representative dan burton openly supporting this movement and holding a number of congressional hearings on the subject. further support for the association between autism and thiomersal appeared in an article by robert f. kennedy jr. in the magazines rolling stone and salon.com alleging a government conspiracy at a cdc meeting to conceal the dangers of thiomersal to protect the pharmaceutical industry, and a book written by david kirby, evidence of harm, dramatizing the lives of parents of autistic children, with both authors participating in media interviews to promote their work and the controversy. although the allegations by kennedy were denied and a us senate committee investigation later found no evidence to substantiate the most serious allegations, the story had already been well publicized by leveraging kennedy's celebrity. salon magazine subsequently amended kennedy's article five times due to factual errors and later retracted it completely on 16 january 2011, stating that the works of critics of the article and evidence of the flaws in the science connecting autism and vaccines undermined the value of the article to the editors. meanwhile, during this time of increased media publicity of the controversy, public health officials and institutions did little to rebut the concerns and speculative theories being offered. media attention and polarization of the debate has also been fueled by personal injury lawyers who took out full-page ads in prominent newspapers and offered financial support for expert witnesses who dissented from the scientific consensus that there is no convincing evidence for a link between thiomersal and autism. paul offit, a leading vaccine researcher and advocate, has said that the media has a tendency to provide false balance by perpetually presenting both sides of an issue even when only one side is supported by the evidence and thereby giving a platform for the spread of misinformation. despite the consensus from experts that there is no link between thiomersal and autism, many parents continue to believe that such a link exists. these parents share the viewpoint that autism is not just treatable, but curable through "biomedical" interventions and have been frustrated by the lack of progress from more "mainline" scientists in finding this cure. instead, they have supported an alternative community of like-minded parents, physicians and scientists who promote this belief. this mindset has taught these parents to challenge the expertise from the mainstream scientific community. parents have also been influenced by an extensive