Thiomersal and vaccines

Based on Wikipedia: Thiomersal and vaccines

In 2011, a medical journal called the vaccine-autism connection "perhaps the most damaging medical hoax of the last 100 years." That's a striking claim, especially when you consider what happened in the century before: the Spanish flu, the thalidomide disaster, the contaminated blood scandals of the 1980s. Yet the authors weren't being hyperbolic. The thiomersal controversy didn't just spread misinformation—it diverted research funding from promising autism investigations, convinced parents to subject their children to dangerous unproven treatments, and contributed to the erosion of public trust in one of medicine's most successful interventions.

This is the story of how a preservative became a boogeyman.

What Thiomersal Actually Is

Thiomersal—spelled thimerosal in the United States—is an organomercury compound that's been used since the 1930s to prevent bacterial and fungal contamination in vaccines. When you're manufacturing millions of vaccine doses, contamination isn't just a theoretical concern. In 1928, a batch of diphtheria vaccine contaminated with Staphylococcus bacteria killed twelve children in Bundaberg, Australia. Preservatives like thiomersal exist to prevent exactly these kinds of tragedies.

The compound works remarkably well at this job. In multi-dose vials—containers designed to provide multiple vaccinations from a single bottle—each needle puncture creates an opportunity for microbes to enter. Thiomersal keeps them from taking hold and multiplying.

Here's where the chemistry gets important. Thiomersal contains mercury, which immediately triggers alarm bells in many people's minds. Mercury is, after all, famously toxic. But chemistry isn't that simple. Table salt contains chlorine, a poison gas used in World War One. Water contains hydrogen, which explodes when ignited. The elements in a compound don't determine its properties—the molecular structure does.

Thiomersal contains ethylmercury, which is chemically distinct from methylmercury—the form of mercury that accumulates in fish and can cause serious harm. This distinction matters enormously. Ethylmercury is processed by the body much more quickly than methylmercury. It doesn't accumulate in tissues the same way. Conflating the two is like confusing ethanol, which humans have been drinking for millennia, with methanol, which causes blindness and death.

The Convergence That Created a Crisis

The thiomersal controversy didn't emerge from nowhere. It arose from the intersection of three separate trends, none of which individually would have sparked a crisis, but together created the perfect conditions for public panic.

The first trend was a heightened awareness of mercury poisoning. Two catastrophic events had seared mercury's dangers into public consciousness. In the 1950s and 1960s, industrial waste dumped into Japan's Minamata Bay caused horrific neurological damage to local residents who ate contaminated fish. Then, in the 1970s, Iraq experienced mass poisonings when wheat treated with mercury-based fungicides was mistakenly used to make bread. These disasters were real, tragic, and well-publicized. By the 1990s, public health agencies were issuing a confusing array of warnings about mercury exposure, particularly for pregnant women. Mercury had become a household word for danger.

The second trend was an expansion of the childhood vaccination schedule. Throughout the 1990s, new vaccines were added to protect children from diseases like Haemophilus influenzae type b, hepatitis B, and pertussis. These were genuine public health advances. But each addition meant more injections, and some of those vaccines contained thiomersal.

The third trend was a dramatic rise in autism diagnoses. More children than ever were being identified as autistic, and parents were understandably searching for explanations.

What most people didn't realize was that the increase in autism diagnoses wasn't necessarily an increase in autism itself. Diagnostic criteria had broadened significantly. Awareness among doctors, teachers, and parents had grown. Services for autistic children had expanded, creating incentives for diagnosis. The age at which children were typically identified had dropped. When you look for something more carefully, you find more of it.

But for parents watching the numbers climb, the pattern seemed clear: more vaccines, more mercury, more autism. It was an elegant theory. It was also wrong.

The Precautionary Principle Backfires

In 1997, the Food and Drug Administration Modernization Act required a review of all mercury-containing products, including vaccines. When officials crunched the numbers, they found something that gave them pause. Under certain circumstances—depending on which vaccines an infant received and how much the infant weighed—a baby's mercury exposure during the first six months of life could exceed Environmental Protection Agency guidelines.

The guidelines in question were designed for methylmercury, not ethylmercury. The two compounds behave differently in the body. There was no evidence that thiomersal at vaccine doses had ever harmed anyone, beyond occasional local reactions at injection sites.

But public health officials faced a dilemma. They could explain the technical differences between types of mercury compounds and hope the public understood. Or they could remove thiomersal from vaccines as a precautionary measure, demonstrating their commitment to safety while eliminating any theoretical risk.

They chose removal.

In July 1999, the United States Public Health Service and the American Academy of Pediatrics issued a joint statement. They emphasized that there was "no evidence of harm caused by doses of thimerosal found in vaccines, except for local hypersensitivity reactions." But they also recommended that vaccine manufacturers phase out thiomersal as quickly as possible.

The intention was to increase public confidence in vaccines. The effect was precisely the opposite. Parents, hearing that thiomersal was being removed, drew the obvious conclusion: it must have been dangerous. Why else would they take it out?

This is the dark side of the precautionary principle. When authorities take action "just to be safe," they implicitly suggest there was something to be safe from. The removal of thiomersal became evidence of its danger, even though the removal was based on the absence of evidence either way.

The Research Verdict

Science doesn't stand still. In the years following the thiomersal controversy's emergence, researchers conducted extensive studies to determine whether there was any connection between the preservative and autism.

The evidence was overwhelming and consistent: there was no link.

First, researchers examined the clinical symptoms. Mercury poisoning and autism present very differently. Mercury poisoning causes specific, recognizable symptoms—tremors, coordination problems, sensory disturbances. Autism manifests as differences in social interaction, communication, and behavior. A child with mercury poisoning doesn't look like a child with autism. A doctor examining both wouldn't confuse them.

Second, epidemiological studies—large population-level analyses—found no correlation between thiomersal exposure and autism rates. Study after study, in country after country, reached the same conclusion. Children who received vaccines with thiomersal were no more likely to develop autism than children who didn't.

Third, and perhaps most tellingly, when thiomersal was removed from most childhood vaccines in the United States by 2001, autism diagnoses didn't decrease. They kept rising. If thiomersal caused autism, removing it should have bent the curve. It didn't.

The scientific consensus became as solid as scientific consensuses get. The Institute of Medicine, the World Health Organization, the Food and Drug Administration, the Centers for Disease Control and Prevention—every major scientific and medical body reached the same conclusion. Thiomersal in vaccines does not cause autism or other neurodevelopmental disorders.

The Controversy That Wouldn't Die

Scientific consensus, unfortunately, doesn't automatically translate to public acceptance. The thiomersal-autism myth proved remarkably resilient.

Part of the explanation lies in how the controversy was promoted. In 2005, Robert F. Kennedy Jr. published an article in Rolling Stone and Salon magazines alleging a government conspiracy to hide thiomersal's dangers. The article claimed that during a 2000 meeting, the Centers for Disease Control and Prevention had discovered a link between thiomersal and autism, then covered it up to protect the pharmaceutical industry.

The allegations were false. A Senate committee investigation found no evidence to support them. Kennedy's article was amended five times to correct factual errors, and Salon eventually retracted it entirely in 2011, acknowledging that the evidence showed the science connecting autism and vaccines was fundamentally flawed.

But by then, the damage was done. Kennedy's celebrity gave the story reach that dry scientific papers couldn't match. The conspiracy narrative appealed to people's distrust of institutions. And once a belief takes hold, it's remarkably difficult to dislodge.

Personal injury lawyers added fuel to the fire, taking out full-page newspaper advertisements and funding expert witnesses willing to dissent from the scientific consensus. Thousands of lawsuits were filed seeking damages from vaccine manufacturers. The cases worked their way through the courts, with plaintiffs arguing that vaccines had caused their children's autism.

The courts, after examining the evidence, ruled against the plaintiffs in multiple representative test cases. The scientific evidence simply wasn't there.

The Infrastructure of Misinformation

Understanding why the thiomersal myth persists requires understanding the ecosystem that sustains it.

Parents of autistic children are, understandably, searching for answers. Autism often appears suddenly, with children who seemed to be developing typically losing skills or failing to meet expected milestones. The timing often coincides roughly with the vaccination schedule—not because vaccines cause autism, but because both typically occur in early childhood. The human brain is wired to find patterns and assign causation. When two events happen near each other, we instinctively assume one caused the other.

Into this emotional landscape stepped an alternative medical community ready to validate parents' fears and offer hope. Online anti-vaccination websites presented themselves as repositories of suppressed truth. Organizations framed the controversy as a battle between caring parents and a corrupt medical establishment. The narrative had heroes and villains, cover-ups and brave truth-tellers. It was compelling in a way that peer-reviewed studies never could be.

Celebrity endorsements amplified the message. Model and actress Jenny McCarthy published a book about her autistic son and appeared on The Oprah Winfrey Show to promote the vaccine-autism connection. Her personal story resonated with audiences in ways that statistical analyses couldn't.

The media, in attempting to be balanced, often made things worse. Vaccine researcher Paul Offit has criticized the tendency to present "both sides" of an issue even when only one side is supported by evidence. This false balance gives fringe views a legitimacy they haven't earned and confuses audiences about where the scientific consensus actually lies.

Real Harm from a False Theory

The thiomersal controversy wasn't just an academic dispute. It caused measurable harm.

Some parents, convinced that mercury had poisoned their autistic children, sought out chelation therapy—a medical procedure that removes heavy metals from the body. Chelation has legitimate uses in actual heavy metal poisoning. But autism isn't caused by heavy metal poisoning, and chelation carries real risks. Studies found that between two and eight percent of children with autism had undergone chelation therapy by 2008. In at least one case, a child died from the treatment.

Meanwhile, fear of thiomersal contributed to broader vaccine hesitancy. Parents, worried about exposing their children to mercury, delayed or refused vaccinations. This eroded the herd immunity that protects communities from outbreaks. Diseases that had been nearly eliminated, like measles, began reappearing.

Perhaps most insidiously, the controversy diverted attention and resources. Money and research effort that could have gone toward understanding autism's actual causes was instead spent repeatedly debunking the same thoroughly disproven theory. Every study confirming that thiomersal doesn't cause autism was a study that couldn't investigate more promising leads.

Where Things Stand Now

Thiomersal has been removed from most childhood vaccines in the United States and the European Union. It remains in some multi-dose flu vaccines, where its preservative properties are still valuable. No vaccines in the European Union currently contain thiomersal as a preservative.

The removal hasn't ended the controversy. In June 2025, the Centers for Disease Control and Prevention, under the leadership of Robert F. Kennedy Jr.—the same Robert F. Kennedy Jr. whose article on thiomersal was retracted fourteen years earlier—restricted thiomersal use even in influenza vaccines. The scientific consensus hasn't changed. But politics has.

Kennedy's organization, Children's Health Defense, continues to promote the claim that thiomersal causes autism. The claim continues to appear in anti-vaccination propaganda, stated as if it were fact rather than a hypothesis that has been tested and rejected by the scientific community.

Lessons from a Manufactured Crisis

The thiomersal controversy offers several uncomfortable lessons about how misinformation spreads and persists.

First, well-intentioned precautionary measures can backfire spectacularly. The removal of thiomersal from vaccines, undertaken to increase public confidence, instead undermined it. When authorities act "just to be safe," they may inadvertently suggest there was danger.

Second, emotional narratives defeat statistical evidence in public discourse. A mother describing her child's diagnosis is more persuasive to most audiences than a meta-analysis of epidemiological studies. This isn't because people are stupid—it's because human brains evolved to learn from stories and personal testimony, not from abstract data.

Third, conspiracy theories are nearly impossible to fully extinguish. Every piece of evidence against the theory becomes evidence of the cover-up. Every official denial proves the conspiracy runs deeper than anyone realized. The unfalsifiable nature of conspiracy thinking makes it remarkably resilient.

Fourth, scientific consensus matters, but consensus alone doesn't win arguments. Scientists are trained to qualify their statements, acknowledge uncertainty, and present findings cautiously. Advocates are free to speak with absolute conviction. In the court of public opinion, certainty often beats nuance.

Finally, the thiomersal controversy illustrates how real suffering can be exploited. Parents of autistic children deserve answers and support. What they got instead was a false explanation that led some to subject their children to dangerous treatments, a distraction that delayed real research progress, and a political football that continues to cause harm decades after the scientific question was settled.

The most damaging medical hoax of the last hundred years wasn't a single lie. It was a perfect storm of legitimate fears, institutional missteps, opportunistic advocacy, and human psychology. Understanding how it happened is the first step toward preventing the next one.