network of anti-vaccination organizations such as robert f. kennedy jr.'s children's health defense and a large number of online anti-vaccination websites that present themselves as an alternative source for evidence using pseudoscientific claims. these websites use emotional appeals to gather support and frame the controversy as an adversarial dispute between parents and a conspiracy of doctors and scientists. advocates for a thiomersal-autism link have also relied on celebrities like model jenny mccarthy and information presented on don imus' imus in the morning radio show to persuade the public to their cause, instead of relying only on "dry" scientific papers and scientists. mccarthy has published a book describing her personal experience with her autistic son and appeared on the oprah winfrey show to promote the hypothesis of vaccines causing autism. bitterness over this issue has led to numerous threats made against the cdc as well as researchers like offit, with increased security placed by the cdc in response to these threats. scientific evaluation rationale for doubting link various lines of evidence undermine a proposed link between thiomersal and autism. for example, although advocates of a thiomersal-autism link consider autism a form of "mercury poisoning," the typical symptoms of mercury toxicity are significantly different from symptoms seen in autism. likewise, the neuroanatomic and histopathologic features of the brains of patients who have mercury poisoning, both with methylmercury as well as ethylmercury, have significant differences from the brains of people with autism. previous episodes of widespread mercury toxicity in a population such as in minamata bay, japan would also be expected to lead to documentation of a significant rise in autism or autism-like behavior in children should autism be caused by mercury poisoning. however, research on several episodes of acute and chronic mercury poisoning have not documented any such rise in autism-like behavior. although some parents cite an association between the timing of onset of autistic symptoms with the timing of vaccinations as evidence of an environmental cause such as thiomersal, this line of reasoning can be misleading. associations such as these do not establish causation as the two occurring together may be only coincidental in nature. also, genetic disorders that have no environmental triggers such as rett syndrome and huntington's disease nevertheless have specific ages when they begin to show symptoms, suggesting specific ages of onset of symptoms does not necessarily require an environmental cause. although the concern for a thiomersal-autism link was originally derived from indirect evidence based on the known potent neurotoxic effects of methylmercury, recent studies show these feared effects were likely overestimated. ethylmercury, such as in thiomersal, clears much faster from the body after administration than methylmercury, suggesting total mercury exposure over time is much less with ethylmercury. currently used methods of estimating brain deposition of mercury likely overestimates the amounts deposited due to ethylmercury, and ethylmercury also decomposes quicker in the brain than methylmercury, suggesting a lower risk of brain damage. these findings show that the assumptions that originally led to concern about the toxicity of ethylmercury, which were based on direct comparison to methylmercury, were flawed. population studies multiple studies have been performed on data from large populations of children to study the relationship between the use of vaccines containing thiomersal, and autism and other neurodevelopmental disorders. almost all of these studies have found no association between thiomersal-containing vaccines (tcvs) and autism, and studies done after the removal of thiomersal from vaccines have nevertheless shown autism rates continuing to increase. the only epidemiologic research that has found a purported link between tcvs and autism has been conducted by mark geier, whose flawed research has not been given any weight by independent reviews. in europe, a cohort study of 467,450 danish children found no association between tcvs and autism or autism spectrum disorders (asds), nor any dose-response relationship between thiomersal and asds that would be suggestive of toxic exposure. an ecological analysis that studied 956 danish children diagnosed with autism likewise did not show an association between autism and thiomersal. a retrospective cohort study on 109,863 children in the united kingdom found no association between tcvs and autism, but a possible increased risk for tics. analysis in this study also showed a possible protective effect with respect to general developmental disorders, attention-deficit disorder, and otherwise unspecified developmental delay. another uk study based on a prospective cohort of 13,617 children likewise found more associated benefits than risks from thiomersal exposure with respect to developmental disorders. because the danish and uk studies involved only diphtheria-tetanus-pertussis (dtp) or diphtheria-tetanus (dt) vaccines, they are less relevant for the higher thiomersal exposure levels that occurred in the u.s. in north america, a canadian study of 27,749 children in quebec showed that thiomersal was unrelated to the increasing trend in pervasive developmental disorders (pdds). in fact, the study noted that rates of pdds were higher in the birth cohorts with no thiomersal when compared to those with medium or high levels of exposure. a study performed in the us which analyzed data from 78,829 children enrolled in hmos taken from the vaccine safety datalink (vsd) did not show any consistent association between tcvs and neurodevelopmental outcomes, noting different results from data in different hmos. a study performed in california found that removal of thiomersal from vaccines did not decrease the rates of autism, suggesting that thiomersal could not be the primary cause of autism. a study on children from denmark, sweden and california likewise argued against tcvs being causally associated with autism. scientific consensus in 2001 the centers for disease control and prevention and the national institutes of health asked the u.s. national academy of sciences' (nas) institute of medicine to establish an independent expert committee to review hypotheses about existing and emerging immunization safety concerns. this initial report found that based on indirect and incomplete evidence available at the time, there was inadequate evidence to accept or reject a thiomersal-autism link, though it was biologically plausible. since this report was released, several independent reviews have examined the body of published research for a possible thiomersal-autism link by examining the theoretical mechanisms of thiomersal causing harm and by reviewing the in vitro, animal, and population studies that have been published. these reviews determined that no evidence exists to establish thiomersal as the cause of autism or other neurodevelopmental disorders. the scientific consensus on the subject is reflected in a follow up report that was subsequently published in 2004 by the institute of medicine, which took into account new data that had been published since the 2001 report. the committee noted, in response to those who cite in vitro or animal models as evidence for the link between autism and thiomersal: however, the experiments showing effects of thimerosal on biochemical pathways in cell culture systems and showing abnormalities in the immune system or metal metabolism in people with autism are provocative; the autism research community should consider the appropriate composition of the autism research portfolio with some of these new findings in mind. however, these experiments do not provide evidence of a relationship between vaccines or thimerosal and autism. in the absence of experimental or human evidence that vaccination (either the mmr vaccine or the preservative thimerosal) affects metabolic, developmental, immune, or other physiological or molecular mechanisms that are causally related to the development of autism, the committee concludes that the hypotheses generated to date are theoretical only. the committee concludes: thus, based on this body of evidence, the committee concludes that the evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism. further evidence of the scientific consensus includes the rejection of a causal link between thiomersal and autism by multiple national and international scientific and medical bodies including the american medical association, the american academy of pediatrics, the american college of medical toxicology, the canadian paediatric society, the u.s. national academy of sciences, the food and drug administration, centers for disease control and prevention, the world health organization, the public health agency of canada, and the european medicines agency. a 2011 journal article reflects this point of view and described the vaccine-autism connection as "the most damaging medical hoax of the last 100 years". consequences the suggestion that thiomersal has contributed to autism and other neurodevelopmental disorders has had a number of effects. public health officials believe fear driven by advocates of a thiomersal-autism link has caused parents to avoid vaccination or adopt "made up" vaccination schedules that expose their children to increased risk from preventable diseases such as measles and pertussis. advocates of a thiomersal-autism link have also helped enact laws in six states (california, delaware, illinois, missouri, new york and washington) between 2004 and 2006 to limit the use of thiomersal given to pregnant women and children, although later attempts in 2009 in twelve other states failed to pass. these laws can be temporarily suspended, but vaccine advocates doubt their utility given the lack of evidence for danger with thiomersal in vaccines. vaccine advocates are also concerned that passage of such laws help fuel a backlash against vaccination and contribute to doubts about the safety of vaccines that are unwarranted. during the period of time of removal of thiomersal, the cdc and aap asked doctors to delay the birth dose of hepatitis b vaccine in children not at risk for hepatitis. this decision, though following the precautionary principle, nevertheless sparked confusion, controversy and some harm. approximately 10% of hospitals suspended the use of hepatitis b vaccine for all newborns, and one child born to a michigan mother infected with hepatitis b virus died of it. similarly, a study found that the number of hospitals who failed to properly vaccinate infants of hepatitis b seropositive mothers rose by over 6 times. this is a potential negative outcome given the high probability that infants who acquire hepatitis b infection at birth will develop the infection in a chronic form and possibly liver cancer. the notion that thiomersal causes autism has led some parents to have their children treated with costly and potentially dangerous therapies such as chelation therapy, which is typically used to treat heavy metal poisoning, due to parental fears that autism is a form of "mercury poisoning". as many as 2 to 8% of autistic children in the u.s., numbering as many as several thousand children per year, receive mercury-chelating agents. although critics of using chelation therapy as an autism treatment point to a lack of any evidence to support its use, hundreds of doctors prescribe these medications despite possible side effects including nutritional deficiencies as well as damage to the liver and kidney. the popularity of this therapy caused a "public health imperative" that led the u.s. national institute of mental health (nimh) to commission a study about chelation in autism by studying dmsa, a chelating agent used for lead poisoning, despite worries from critics that there would be no chance it would show positive results and it would be unlikely to convince parents to not use the therapy. ultimately, the study was halted due to ethical concerns that there would be too much risk to children with autism who did not have toxic levels of mercury or lead due to a new animal study showing possible cognitive and emotional problems associated with dmsa. a 5-year-old autistic boy died from cardiac arrest immediately after receiving chelation therapy treatment using edta in 2005. the notion has also diverted attention and resources away from efforts to determine the causes of autism. the 2004 institute of medicine report committee recommended that while it supported "targeted research that focuses on better understanding the disease of autism, from a public health perspective the committee does not consider a significant investment in studies of the theoretical vaccine-autism connection to be useful at this time." alison singer, a senior executive of autism speaks, resigned from the group in 2009 in a dispute over whether to fund more research on links between vaccination and autism, saying, "there isn't an unlimited pot of money, and every dollar spent looking where we know the answer isn't is one less dollar we have to spend where we might find new answers." court cases further information: vaccine court further information: autism omnibus trial from 1988 until august 2010, 5,632 claims relating to autism were made to office of special masters of the u.s. court of federal claims