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Thiomersal and vaccines

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This article uses the International Nonproprietary Name "thiomersal", not the common U.S. spelling "thimerosal".


The thiomersal controversy is between those who claim that vaccines containing the mercury-based preservative thiomersal contribute to the development of autism and other brain development disorders[1] and those who support the current scientific consensus that there is no convincing scientific evidence supporting these claims.[2][3]

Thiomersal, is an organomercury compound used as a preservative in vaccines since the 1930s to prevent bacterial and fungal contamination.[4][5] In July 1999, following a review of mercury-containing food and drugs, the Centers for Disease Control (CDC) and the American Academy of Pediatrics (AAP) asked vaccine makers to remove thiomersal from vaccines as quickly as possible as a purely precautionary measure, and it was rapidly phased out of most U.S. and European vaccines.[6][7] Many parents took the action to remove thiomersal as indicating that the preservative was harmful, and there have been thousands of lawsuits filed in the U.S. to seek damages from alleged toxicity from vaccines, including those purportedly caused by thiomersal.[6][8]

Major scientific and medical bodies such as the Institute of Medicine and World Health Organization[9] as well as governmental agencies such as the Food and Drug Administration[4] and the CDC[10] reject any role for thiomersal in autism or other neurodevelopmental disorders. Multiple lines of scientific evidence have been cited to support this conclusion: for example, the clinical symptoms of mercury poisoning differ significantly from those of autism.[11] Most conclusively, eight major studies (as of 2008) examined the effect of reductions or removal of thiomersal from vaccines. All eight demonstrated that autism rates failed to decline despite removal of thiomersal, arguing strongly against a causative role.[12][13][14][15][16][17][18][19]

Reasons for opposing thiomersal in vaccines

Scientific background

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 to provide detailed information about the thimerosal content of their preparations in December 1998 and April 1999.[20] Upon conclusion of this 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 American Academy of Pediatrics (AAP), concluded:

"Our review revealed no evidence of harm caused by doses of thimerosal found in vaccines, except for local hypersensitivity reactions. At the time of our review, vaccines containing thimerosal as a preservative could expose infants to cumulative mercury at levels that exceed EPA recommendations during the first 6 months of life. The clinical significance of this conclusion is not currently known; EPA guidelines contain as much as a 10-fold safety factor and such guidelines are meant to be starting points for the evaluation of mercury exposure. However, reducing exposure to thimerosal from vaccines is merited given the goal of reducing human exposure to mercury from all sources, the feasibility of removing thimerosal as a vaccine preservative, and the desirability of ensuring public confidence in the safety of vaccines."[21]

The FDA noted that while the vaccination schedule at that time might have exceeded EPA standards for mercury exposure during the first 6 months of life, it did not exceed those of the FDA, Agency for Toxic Substances and Disease Registry (ATSDR), or WHO. The FDA also noted 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. 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.[4] 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.[2]

Despite these efforts to head off concern about vaccination, public fears about thiomersal continued to increase, especially among parents caring for autistic children, some of whom suspected a role of thiomersal containing vaccines in causing autism. Therefore, the Centers for Disease Control and the National Institutes of Health (NIH) asked the National Academy of Science's (NAS) Institute of Medicine (IOM) to establish an independent expert committee to review hypotheses about existing and emerging immunization safety concerns. In 2001 the committee reported:

"The committee concludes that although the hypothesis that exposure to thimerosal-containing vaccines could be associated with neurodevelopmental disorders is not established and rests on indirect and incomplete information, primarily from analogies with methylmercury and levels of maximum mercury exposure from vaccines given in children, the hypothesis is biologically plausible. The committee also concludes that the evidence is inadequate to accept or reject a causal relationship between thimerosal exposures from childhood vaccines and the neurodevelopmental disorders of autism, ADHD, and speech or language delay."[22]

Political support for removal

The FDA actions prompted autism advocates to consider the possibility of thiomersal as a cause of autism. The indirect evidence included analogy with neurotoxic effects of mercury compounds, extrapolation from laboratory and animal studies, and comparisons between trends in vaccination and autism cases.[23]

This concept also gained support in the political sphere, with Rep. Dan Burton and Rep. David Weldon openly supporting this movement as well. A number of hearings were held in the Subcommittee on Human Rights and Wellness, Committee on Government Reform, chaired by Rep. Burton, on the topic of autism and vaccines. His staff concluded:

"Thimerosal used as a preservative in vaccines in [sic] likely related to the autism epidemic. This epidemic in all probability may have been prevented or curtailed had the FDA not been asleep at the switch regarding the lack of safety data regarding injected thimerosal and the sharp rise of infant exposure to this known neurotoxin. Our public health agencies’ failure to act is indicative of institutional malfeasance for self-protection and misplaced protectionism of the pharmaceutical industry"[1]

Rationale for concern

Supporters of a link between thiomersal and autism cite several lines of reasoning for their concerns, including:

  • Appeal for caution: precise thresholds for ethylmercury toxicity have not been fully studied, and methylmercury is a poor surrogate for studying the toxicity of thiomersal.[4][24]
  • In vitro tests: cultured cells in laboratory show adverse effects when exposed to ethylmercury.[25]
  • An in vivo study reported in 2004 that autoimmune disease-sensitive mice exposed to thiomersal had growth delay, reduced locomotion, exaggerated response to new stimuli, and uncommon neuronal structure in some brain areas. The exposure attempted to replicate the one-year schedule for U.S. infants in 2001, adjusted for weight and age.[26] However, these results could not be replicated; a 2007 study by another group using the same mouse strain found no pervasive developmental neurotoxicity.[27] Also, a 2009 study reported that male rats are more susceptible to thiomersal poisoning than female,[28].
  • Unscientific reports that autism is rarer in the Amish community and other non-vaccinated groups. The reports are undercut by the fact that Amish genes may differ from those in the general community, that people in the Amish community have low exposures to many other potential hazards (for example, pesticides and plastics) and that increasingly, the Amish do receive at least some vaccinations.[29]
Bar chart versus time. The graph rises steadily from 1996 to 2007, from about 0.7 to about 5.3. The trend curves slightly upward.
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.[30]
  • Epidemiologic data: a series of epidemiologic studies coauthored by Mark Geier claimed a population-level correlation between thiomersal and autism.[31] An article in the journal Pediatrics found these epidemiologic studies to suffer from numerous fundamental methodological flaws which bias and invalidate their claims.[32] 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;[33] it is unknown whether autism's true prevalence increased during the period.[30]
  • Concern that mercury might have synergistic effects with other metals and toxicants.[34]

Scientific rejection of danger

Another committee report commissioned by the CDC by the Institute of Medicine follows up on the initial 2001 report. Since the 2001 report, the IOM committee took into account new data that had been published in the interim, including a number of large scale epidemiologic studies focusing on the relationship between thiomersal and autism in a number of countries including the US, Sweden, Denmark, and the UK.

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."[35]

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."[35] [bold in original]

This committee felt so strongly about this conclusion that they state:

"The committee concludes that much more research must be conducted on autism. However, research should be directed towards those lines of inquiry most supported by the current state of knowledge. The vaccine hypotheses are not currently supported by the evidence."[35]

Three large-scale controlled observational studies have been reported on this issue; none have found an association between thiomersal-containing vaccines (TCVs) and autism.[23] A study from Denmark noted no decrease in autism rates despite cessation of TCVs[15] and a UK study found that TCVs actually had a protective effect with respect to autism.[16] 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.[23] For the U.S., a study based on the Vaccine Safety Datalink found no association between TCVs and autism.[36] Some smaller studies have also found no association between TCVs and autism[13][14] and a study found no association between thimerosal and the neurological signs of autism.[12] Another smaller study also found a protective effect: it reported a significantly lower prevalence of autism spectrum disorders among children exposed to thimerosal (5.95 per 1,000 versus 8.27 per 1,000).[17]

The research of Mark Geier, the main source of epidemiologic data used by supporters of a link between thiomersal and autism, has received considerable criticism,[37] including charges of not presenting methods and statistical analyses to others for verification,[32] improperly analyzing data taken from Vaccine Adverse Event Reporting System,[32][35] as well as either mislabelling or confusing fundamental statistical terms in his papers.[35]

Further evidence of the position of the scientific consensus includes the rejection of a causal link between thimerosal and autism by the main scientific and medical professional bodies including the American Medical Association,[38] the American Academy of Pediatrics,[39] the American College of Medical Toxicology,[40] the Canadian Paediatric Society,[41] the National Academy of Sciences,[35] the Food and Drug Administration,[4] Centers for Disease Control and Prevention,[10] the World Health Organization,[9] the Public Health Agency of Canada,[42] and the European Medicines Agency.[43]

Removal from vaccines

In July 1999 the CDC and the AAP asked vaccine makers to remove thiomersal from vaccines as quickly as possible, and asked doctors to delay the birth dose of hepatitis B vaccine in children not at risk for hepatitis. The CDC and the AAP followed the precautionary principle, which assumes that there is no harm in exercising caution even if it later turns out to be unwarranted, but their action sparked confusion, controversy and some harm. About 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.[2]

Although thiomersal was removed from routine infant vaccines by 2001 in the U.S.,[6] some vaccines continue to contain non-trace amounts of thiomersal, mainly in multi-dose vaccines targeted against influenza and tetanus.[44] Other products that may contain thimerosal include products derived from blood plasma such as Rho(D) Immune Globulin, and antivenoms for pit viper, coral snake and black widow spider.[45]

The notion that thiomersal causes autism has led some parents to have their children treated with chelation therapy, with as many as 2 to 8% of autistic children in the U.S., numbering as many as several thousand children per year, receiving mercury-chelating agents.[46] A 5-year-old autistic boy died from cardiac arrest immediately after receiving chelation therapy treatment using EDTA in 2005.[47] In 2007, the U.S. National Institute of Mental Health (NIMH) halted an experimental trial that gave the chelating agent DMSA to children with autism who did not have toxic blood level mercury or lead due to ethical concerns about safety after results of DMSA effects in rats were published.[46]

The notion has also diverted attention and resources away from efforts to determine the causes of autism.[2] 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."[48]

Court cases

From 1988 until August 2009, 5,600 claims relating to autism were made to Office of Special Masters of the U.S. Court of Federal Claims (commonly known as the "Vaccine Court") which oversees vaccine injury claims, of which 555 cases have been dismissed and no compensations made, with the remaining cases pending.[49] In one case, the U.S. government agreed to pay for injury to a child that had a pre-existing mitochondrial disorder who developed autism-like symptoms after multiple vaccinations, some of which included thiomersal. Citing the inability to rule out a role of these vaccinations in exacerbated her underlying mitochondrial disorder as the rationale for payment, CDC officials cautioned against generalizing this one case to all autism-related vaccine cases as most patients with autism do not have a mitochondrial disorder.[50][51] In February 2009, this court also ruled on three autism-related cases, each exploring different mechanisms that plaintiffs proposed linked thiomersal-containing vaccines with autism. Three judges independently found no evidence that vaccines caused autism and denied the plaintiffs compensation. Since these same mechanisms formed the basis for the vast majority of remaining autism-related vaccine injury cases, the chance for compensation in any of these cases have significantly decreased.[52]

References

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