Thimerosal is a mercury-containing compound that has been a subject of public discussion, particularly concerning a possible link to autism. Public concern arose regarding its use in vaccines and whether it might contribute to developmental conditions. This article explores what thimerosal is, how the hypothesis linking it to autism began, the scientific consensus on the matter, and its current presence in vaccines.
What Thimerosal Is
Thimerosal is an organic compound containing ethylmercury, used as a preservative in medical products since the 1930s. In vaccines, its primary function is to prevent the growth of harmful bacteria and fungi that could be introduced into multi-dose vials. This helps ensure vaccine safety and effectiveness, especially when multiple doses are drawn from a single container. A vaccine with 0.01% thimerosal contains approximately 25 micrograms of mercury per 0.5 mL dose, comparable to the amount of elemental mercury in a 3-ounce can of tuna.
How the Thimerosal-Autism Hypothesis Began
The hypothesis linking thimerosal to autism emerged in the 1990s amidst a perceived increase in autism diagnoses and public unease about mercury exposure. Environmental agencies had identified that mercury could have neurotoxic effects, particularly methylmercury found in fish, which contributed to broader concerns. Initial theories and anecdotal reports, often lacking rigorous scientific methodology, began to connect the mercury content in thimerosal with the rising rates of autism.
This public anxiety, rather than definitive scientific proof, prompted a precautionary response from public health agencies. In 1999, despite no established evidence of harm from thimerosal in vaccines, public health agencies, including the Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics (AAP), along with vaccine manufacturers, agreed to reduce or eliminate thimerosal from many childhood vaccines. This action was a proactive measure to minimize overall mercury exposure during early development.
Scientific Consensus on Thimerosal and Autism
Extensive scientific research has been conducted, consistently finding no causal relationship between thimerosal and autism. Numerous large-scale epidemiological studies, conducted in various countries like the United States, the United Kingdom, Denmark, and Sweden, have examined this hypothesis. These studies have compared vaccinated and unvaccinated children, or groups exposed to different levels of thimerosal, and have not found an increased risk of autism spectrum disorder.
Major health organizations globally, including the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), the Institute of Medicine (IOM), and the American Academy of Pediatrics (AAP), have reviewed the available evidence. They have all concluded that the evidence favors rejecting a causal relationship between thimerosal-containing vaccines and autism. For example, a study in Denmark of 956 children diagnosed with autism between 1971 and 2000 found no increased risk when thimerosal was included in vaccines, and autism incidence continued to rise even after its removal.
Furthermore, the clinical symptoms of mercury poisoning differ significantly from those of autism, providing additional evidence against a link. Studies have also shown that ethylmercury, the type of mercury in thimerosal, is processed and cleared from the human body more quickly than methylmercury, the form of mercury found in certain fish that is known to be toxic at high levels. This difference in how the body handles the two forms of mercury further supports the scientific consensus that thimerosal in vaccines does not cause autism.
Thimerosal in Today’s Vaccines
Thimerosal was removed from most routine childhood vaccines in the United States by 2001, primarily as a precautionary measure due to public concern rather than scientific evidence of harm. For instance, measles, mumps, and rubella (MMR) vaccines have never contained thimerosal, nor have varicella (chickenpox), inactivated polio (IPV), or pneumococcal conjugate vaccines.
Despite its removal from most childhood vaccines, thimerosal is still present in some multi-dose vials of influenza (flu) vaccine and certain other vaccines globally. For the 2024-2025 respiratory virus season, approximately 96% of administered influenza vaccines in the U.S. did not contain thimerosal, with thimerosal-free options widely available for those who prefer them.