Thiomersal And Vaccines
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Thiomersal Thiomersal (INN), or thimerosal (USAN, JAN), is an organomercury compound. It is a well-established antiseptic and antifungal agent. The pharmaceutical corporation Eli Lilly and Company gave thiomersal the trade name Merthiolate. It has been u ...
(or thimerosal) is a
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
compound which is used as a preservative in some
vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifie ...
s.
Anti-vaccination Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain abou ...
activists promoting the incorrect claim that vaccination causes
autism The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...
have asserted that the mercury in thiomersal is the cause. There is no
scientific evidence Scientific evidence is evidence that serves to either support or counter a scientific theory or hypothesis, although scientists also use evidence in other ways, such as when applying theories to practical problems. "Discussions about empirical ev ...
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 Mercury poisoning is a type of metal poisoning due to exposure to mercury. Symptoms depend upon the type, dose, method, and duration of exposure. They may include muscle weakness, poor coordination, numbness in the hands and feet, skin rashe ...
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 The National Academy of Medicine (NAM), formerly called the Institute of Medicine (IoM) until 2015, is an American nonprofit, non-governmental organization. The National Academy of Medicine is a part of the National Academies of Sciences, E ...
and
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...
(WHO) as well as governmental agencies such as the
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respon ...
(FDA) and the
Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
(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. Robert Francis Kennedy Jr. (born January 17, 1954) is an American environmental lawyer and author known for promoting anti-vaccine propaganda and conspiracy theories. Kennedy is a son of U.S. senator Robert F. Kennedy and a nephew of President ...
, through his group
Children's Health Defense Children's Health Defense is an American 501(c)(3) nonprofit activist group mainly known for anti-vaccine propaganda and has been identified as one of the main sources of misinformation on vaccines. Founded under the name World Mercury Project ...
. 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 A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal deve ...
, the
scientific consensus Scientific consensus is the generally held judgment, position, and opinion of the majority or the supermajority of scientists in a particular field of study at any particular time. Consensus is achieved through scholarly communication at confe ...
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 Organomercury refers to the group of organometallic compounds that contain mercury. Typically the Hg–C bond is stable toward air and moisture but sensitive to light. Important organomercury compounds are the methylmercury(II) cation, CH3Hg+; ...
compound used as a preservative in vaccines to prevent bacterial and
fungal A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
contamination. Following a mandated review of mercury-containing food and drugs in 1999, the
Centers for Disease Control The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
and Prevention (CDC) and the
American Academy of Pediatrics The American Academy of Pediatrics (AAP) is an American professional association of pediatricians, headquartered in Itasca, Illinois. It maintains its Department of Federal Affairs office in Washington, D.C. Background The Academy was founded ...
(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 An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
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 The United States Food and Drug Administration Modernization Act of 1997 (FDAMA) amended the Federal Food, Drug, and Cosmetic Act. This act is related to the regulation of food, drugs, devices, and biological products by the FDA. These changes ...
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 A vaccination schedule is a series of vaccinations, including the timing of all doses, which may be either recommended or compulsory, depending on the country of residence. A vaccine is an antigenic preparation used to produce active immunity to ...
for infants did not exceed FDA,
Agency for Toxic Substances and Disease Registry The Agency for Toxic Substances and Disease Registry (ATSDR) is a federal public health agency within the United States Department of Health and Human Services. The agency focuses on minimizing human health risks associated with exposure to haza ...
(ATSDR), or WHO guidelines on mercury exposure, it could have exceeded
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
(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 Ethylmercury (sometimes ethyl mercury) is a cation composed of an organic CH3CH2- species (an ethyl group) bound to a mercury(II) centre, making it a type of organometallic cation, and giving it a chemical formula C2H5Hg+. The main source of eth ...
in thiomersal because guidelines for mercury toxicity were based primarily on studies of
methylmercury Methylmercury (sometimes methyl mercury) is an organometallic cation with the formula . It is the simplest organomercury compound. Methylmercury is extremely toxic, and its derivatives are the major source of organic mercury for humans. It is a ...
, 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 Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating ex ...
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 In public relations and communication science, publics are groups of individual people, and the public (a.k.a. the general public) is the totality of such groupings. This is a different concept to the sociological concept of the ''Öffentlichkei ...
(USPHS), the
National Institutes of Health The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the late ...
(NIH), CDC and
Health Resources and Services Administration The Health Resources and Services Administration (HRSA) is an agency of the U.S. Department of Health and Human Services located in North Bethesda, Maryland. It is the primary federal agency for improving access to health care services for peop ...
(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 The precautionary principle (or precautionary approach) is a broad epistemological, philosophical and legal approach to innovations with potential for causing harm when extensive scientific knowledge on the matter is lacking. It emphasizes caut ...
, 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 Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...
,
meningococcal disease Meningococcal disease describes infections caused by the bacterium ''Neisseria meningitidis'' (also termed meningococcus). It has a high mortality rate if untreated but is vaccine-preventable. While best known as a cause of meningitis, it can al ...
and
tetanus Tetanus, also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'', and is characterized by muscle spasms. In the most common type, the spasms begin in the jaw and then progress to the rest of the body. Each spasm usually ...
. In 2004
Quackwatch Quackwatch is a United States-based website, self-described as a "network of people" founded by Stephen Barrett, which aims to "combat health-related frauds, myths, fads, fallacies, and misconduct" and to focus on "quackery-related information tha ...
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. , between 2–8% of children with autism had undergone the therapy.


Rationale for concern

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 The ''Haemophilus influenzae'' type B vaccine, also known as Hib vaccine, is a vaccine used to prevent ''Haemophilus influenzae'' type b (Hib) infection. In countries that include it as a routine vaccine, rates of severe Hib infections have d ...
, DTaP vaccine and
hepatitis B vaccine Hepatitis B vaccine is a vaccine that prevents hepatitis B. The first dose is recommended within 24 hours of birth with either two or three more doses given after that. This includes those with poor immune function such as from HIV/AIDS and th ...
, 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 Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specificall ...
effects of other mercury compounds, the reported epidemiologic association between autism and vaccine use, and extrapolation from ''
in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
'' experiments and animal studies. Studies conducted by
Mark Geier Mark R. Geier (born 1948) is an American former physician and controversial professional witness who testified in more than 90 cases regarding allegations of injury or illness caused by vaccines. Since 2011, Geier's medical license has been su ...
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 The Vaccine Adverse Event Reporting System (VAERS) is a United States program for vaccine safety, co-managed by the U.S. Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA). VAERS is a postmarketing surveil ...
, 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 ''Medical Hypotheses'' is a not-conventionally-peer reviewed medical journal published by Elsevier. It was originally intended as a forum for unconventional ideas without the traditional filter of scientific peer review, "as long as (the ideas) are ...
'', a non-
peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work (peers). It functions as a form of self-regulation by qualified members of a profession within the relevant field. Peer review ...
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 Danny Lee Burton (born June 21, 1938) is an American politician. Burton is the former U.S. Representative for , and previously the , serving from 1983 until 2013. He is a member of the Republican Party and was part of the Tea Party Caucus. Ear ...
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. Robert Francis Kennedy Jr. (born January 17, 1954) is an American environmental lawyer and author known for promoting anti-vaccine propaganda and conspiracy theories. Kennedy is a son of U.S. senator Robert F. Kennedy and a nephew of President ...
in the magazines ''
Rolling Stone ''Rolling Stone'' is an American monthly magazine that focuses on music, politics, and popular culture. It was founded in San Francisco, San Francisco, California, in 1967 by Jann Wenner, and the music critic Ralph J. Gleason. It was first kno ...
'' and ''
Salon.com ''Salon'' is an American politically progressive/ liberal news and opinion website created in 1995. It publishes articles on U.S. politics, culture, and current events. Content and coverage ''Salon'' covers a variety of topics, including re ...
'' alleging a government conspiracy at a CDC meeting to conceal the dangers of thiomersal to protect the
pharmaceutical industry The pharmaceutical industry discovers, develops, produces, and markets drugs or pharmaceutical drugs for use as medications to be administered to patients (or self-administered), with the aim to cure them, vaccinate them, or alleviate symptoms. ...
, 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 lawyer A personal injury lawyer is a lawyer who provides legal services to those who claim to have been injured, physically or psychologically, as a result of the negligence of another person, company, government agency or any entity. Personal injury la ...
s who took out full-page ads in prominent newspapers and offered financial support for
expert witness An expert witness, particularly in common law countries such as the United Kingdom, Australia, and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as ...
es who dissented from the scientific consensus that there is no convincing evidence for a link between thiomersal and autism.
Paul Offit Paul Allan Offit (born March 27, 1951) is an American pediatrician specializing in infectious diseases, vaccines, immunology, and virology. He is the co-inventor of a rotavirus vaccine. Offit is the Maurice R. Hilleman Professor of Vaccinology, ...
, a leading vaccine researcher and advocate, has said that the media has a tendency to provide
false balance False balance, also bothsidesism, is a media bias in which journalists present an issue as being more balanced between opposing viewpoints than the evidence supports. Journalists may present evidence and arguments out of proportion to the act ...
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 network of anti-vaccination organizations such as
Robert F. Kennedy Jr. Robert Francis Kennedy Jr. (born January 17, 1954) is an American environmental lawyer and author known for promoting anti-vaccine propaganda and conspiracy theories. Kennedy is a son of U.S. senator Robert F. Kennedy and a nephew of President ...
's
Children's Health Defense Children's Health Defense is an American 501(c)(3) nonprofit activist group mainly known for anti-vaccine propaganda and has been identified as one of the main sources of misinformation on vaccines. Founded under the name World Mercury Project ...
and a large number of online
anti-vaccination Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain abou ...
websites that present themselves as an alternative source for evidence using
pseudoscientific Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claim ...
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 Jennifer McCarthy Wahlberg (' Jennifer Ann McCarthy; born November 1, 1972) is an American actress, model, and television personality. She began her career in 1993 as a nude model for ''Playboy'' magazine and was later named their Playmate of ...
and information presented on
Don Imus John Donald Imus Jr. (July 23, 1940 – December 27, 2019), also known mononymously as Imus, was an American radio personality, television show host, recording artist, and author. His radio show, ''Imus in the Morning'', was aired on various stat ...
' ''
Imus in the Morning ''Imus in the Morning'' was a long-running radio show hosted by Don Imus. The show originated on June 2, 1968, on various stations in the Western United States and Cleveland, Ohio before settling on WNBC radio in New York City in 1971. In Octob ...
'' 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 ''The Oprah Winfrey Show'', often referred to as ''The Oprah Show'' or simply ''Oprah'', is an American daytime broadcast syndication, syndicated talk show that aired nationally for 25 seasons from September 8, 1986, to May 25, 2011, in Chicag ...
'' 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 Histopathology (compound of three Greek words: ''histos'' "tissue", πάθος ''pathos'' "suffering", and -λογία ''-logia'' "study of") refers to the microscopic examination of tissue in order to study the manifestations of disease. Spec ...
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 Minamata is a small factory town. Minamata Bay is a bay on the west coast of Kyūshū island, located in Kumamoto Prefecture, Japan. The bay is part of the larger Shiranui Sea which is sandwiched between the coast of the Kyūshū mainland and the ...
, 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 Rett syndrome (RTT) is a genetic disorder that typically becomes apparent after 6–18 months of age and almost exclusively in females. Symptoms include impairments in language and coordination, and repetitive movements. Those affected often h ...
and
Huntington's disease Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...
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 Cohort or cohortes may refer to: * Cohort (educational group), a group of students working together through the same academic curriculum * Cohort (floating point), a set of different encodings of the same numerical value * Cohort (military unit ...
study of 467,450 Danish children found no association between TCVs and autism or
autism spectrum disorders The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental disorder, neurodevelopmental condition (or conditions) ...
(ASDs), nor any dose-response relationship between thiomersal and ASDs that would be suggestive of toxic exposure. An
ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
analysis that studied 956 Danish children diagnosed with autism likewise did not show an association between autism and thiomersal. A
retrospective cohort study A retrospective cohort study, also called a historic cohort study, is a longitudinal cohort study used in medical and psychological research. A cohort of individuals that share a common exposure factor is compared with another group of equival ...
on 109,863 children in the United Kingdom found no association between TCVs and autism, but a possible increased risk for
tics A tic is a sudden, repetitive, nonrhythmic motor movement or vocalization involving discrete muscle groups.American Psychiatric Association (2000)DSM-IV-TR: Tourette's Disorder.''Diagnostic and Statistical Manual of Mental Disorders'', 4th ed., ...
. Analysis in this study also showed a possible ''protective'' effect with respect to general developmental disorders,
attention-deficit disorder Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inap ...
, 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 The diagnostic category pervasive developmental disorders (PDD), as opposed to specific developmental disorders (SDD), is a group of disorders characterized by delays in the development of multiple basic functions including socialization and co ...
(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
HMO In the United States, a health maintenance organization (HMO) is a medical insurance group that provides health services for a fixed annual fee. It is an organization that provides or arranges managed care for health insurance, self-funded heal ...
s taken from the
Vaccine Safety Datalink The Vaccine Safety Datalink Project (VSD) was established in 1990 by the United States Centers for Disease Control and Prevention (CDC) to study the adverse effects of vaccines. Four large health maintenance organizations, including Kaiser Perman ...
(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 The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
and the
National Institutes of Health The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the late ...
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 ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
'' or
animal models A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the working ...
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. old in original/blockquote> 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 Medical Association (AMA) is a professional association and lobbying group of physicians and medical students. Founded in 1847, it is headquartered in Chicago, Illinois. Membership was approximately 240,000 in 2016. The AMA's state ...
, the
American Academy of Pediatrics The American Academy of Pediatrics (AAP) is an American professional association of pediatricians, headquartered in Itasca, Illinois. It maintains its Department of Federal Affairs office in Washington, D.C. Background The Academy was founded ...
, the American College of Medical Toxicology, the Canadian Paediatric Society, the U.S. National Academy of Sciences, the
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respon ...
,
Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
, the
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...
, the
Public Health Agency of Canada The Public Health Agency of Canada (PHAC; french: Agence de la santé publique du Canada, ASPC) is an agency of the Government of Canada that is responsible for public health, emergency preparedness and response, and infectious and chronic diseas ...
, and the
European Medicines Agency The European Medicines Agency (EMA) is an agency of the European Union (EU) in charge of the evaluation and supervision of medicinal products. Prior to 2004, it was known as the European Agency for the Evaluation of Medicinal Products or Euro ...
. 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 Measles is a highly contagious infectious disease caused by measles virus. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than , cough, ...
and
pertussis Whooping cough, also known as pertussis or the 100-day cough, is a highly contagious bacterial disease. Initial symptoms are usually similar to those of the common cold with a runny nose, fever, and mild cough, but these are followed by two or ...
. 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 Hepatitis B is an infectious disease caused by the ''Hepatitis B virus'' (HBV) that affects the liver; it is a type of viral hepatitis. It can cause both acute and chronic infection. Many people have no symptoms during an initial infection. Fo ...
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 Serostatus refers to the presence or absence of a serological marker in the blood. The presence of detectable levels of a specific marker within the serum is considered seropositivity, while the absence of such levels is considered seronegativity. ...
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 Liver cancer (also known as hepatic cancer, primary hepatic cancer, or primary hepatic malignancy) is cancer that starts in the liver. Liver cancer can be primary (starts in liver) or secondary (meaning cancer which has spread from elsewhere to th ...
. 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 Chelation therapy is a medical procedure that involves the administration of Chelation, chelating agents to remove heavy metals from the body. Chelation therapy has a long history of use in clinical toxicology and remains in use for some very sp ...
, which is typically used to treat
heavy metal poisoning A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts. The term has particular application to cadmium, mercury and lead, all of which appear in the World He ...
, 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 The National Institute of Mental Health (NIMH) is one of 27 institutes and centers that make up the National Institutes of Health (NIH). The NIH, in turn, is an agency of the United States Department of Health and Human Services and is the prima ...
(NIMH) to commission a study about chelation in autism by studying
DMSA Succimer, sold under the brand name Chemet among others, is a medication used to treat lead, mercury, and arsenic poisoning. When radiolabeled with technetium-99m, it is used in a number of types of diagnostic testing. A full course is 19 days ...
, 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 Autism Speaks Inc. is a controversial autism advocacy organization and the largest autism research organization in the United States. It sponsors autism research and conducts awareness and outreach activities aimed at families, governments, an ...
, 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

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 (commonly known as the "Vaccine Court") which oversees vaccine injury claims, of which one case has received compensation, 738 cases have been dismissed with no compensations made, and with the remaining cases pending. In the one case which received compensation, the U.S. government agreed to pay for injury to a child that had a pre-existing
mitochondrial disorder Mitochondrial disease is a group of disorders caused by mitochondrial dysfunction. Mitochondria are the organelles that generate energy for the cell and are found in every cell of the human body except red blood cells. They convert the energy of ...
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 exacerbating 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. 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 has significantly decreased. In March 2010, the court ruled in three other test cases that thiomersal-containing vaccines do not cause autism.


See also

*
Vaccine shedding Vaccine shedding is a form of viral shedding which can occur following a viral infection caused by an attenuated (or "live virus") vaccine, which is a specific vaccine technology that uses an attenuated form of a live virus. Illness in others res ...
*
Vaccine hesitancy Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain abou ...
* Folk epidemiology of autism * * * *
Lancet MMR autism fraud The ''Lancet'' MMR autism fraud centered on the publication in February 1998 of a fraudulent research paper titled "Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children" in ''The Lancet''. T ...


References

{{Vaccine safety Mercury (element)