Thimerosal: What should we know?
At its first meeting in its new composition, the U.S. Advisory Committee on Immunization Practices (ACIP) recommended that influenza vaccines be administered only in formulations that do not contain the mercury-based preservative, thimerosal.
The appointment of Kennedy as U.S. Secretary of Health has shaken the American healthcare system, with its effects rippling across international health organizations. Prominent officials have been dismissed or resigned abruptly. These developments inevitably impact major public health topics.
What is Thimerosal?
Thimerosal is a mercury-based preservative (ethylmercury compound) used in the vaccine industry to prevent bacterial and fungal contamination, particularly in multi-dose vials. For years, it has been a central issue for vaccine opponents who claim it is hazardous, while scientific counterclaims argue it is non-toxic at the doses used. Thimerosal was introduced in the 1930s, patented in 1927, and marketed by Eli Lilly under the name Merthiolate.
Following a tragic 1928 incident in Australia (Bundaberg tragedy), where 12 children died from Staphylococcal contamination of a diphtheria vaccine, the U.S. FDA began mandating preservatives in multi-dose vaccines.
At high doses, mercury and mercury compounds (including methyl- and ethyl-mercury) are known to be nephro- and neurotoxic.
As a precaution, thimerosal was mostly removed from routine pediatric vaccines in the U.S. starting in 2001. All vaccines routinely recommended for children under age 6 in the U.S. are now thimerosal-free.
Thimerosal and Autism Claims:
Claims of a link between thimerosal and autism have been thoroughly investigated and found to lack scientific basis.
Toxicity Research:
While thimerosal is generally regarded as safe, recent in vivo and in vitro studies have explored potential neurotoxic effects. A 2022 scientific review concluded that only a small number of animal studies suggest neurotoxicity, and the broader research base does not support a causal relationship between thimerosal and autism.
Epidemiological Studies: Numerous large-scale epidemiological studies in the U.S., Europe, and elsewhere found no increased autism rates among children vaccinated with thimerosal-containing vaccines compared to those who received thimerosal-free versions. Meta-analyses and systematic reviews strongly support this conclusion.
Which Vaccines Still Contain Thimerosal? Only a limited number of vaccines, primarily multi-dose influenza vaccines, contain thimerosal today:
-Fluzone (Sanofi Pasteur)
-lucelvax (Seqirus)
-Afluria (Seqirus)
Single-dose pre-filled syringes of these vaccines are thimerosal-free.
The FDA estimates that about 4% of the US flu vaccine supply is available as multidose vials.
Some Td (tetanus-diphtheria) vaccines may use thimerosal in manufacturing (e.g., MassBiologics Td), but only trace residual amounts remain. Alternatives such as Tdap are available without thimerosal.
Other than these, no pediatric vaccines in the U.S. contain thimerosal, they are all distributed in single-dose units.
Since 2004, no routine childhood vaccine in the EU contains thimerosal as a preservative. However, some combination vaccines (e.g., Infanrix Hexa, Infanrix Penta) may have residual trace amounts from the production process.
Why Was Thimerosal Removed if It's Safe?
According to the World Health Organization, the removal of thimerosal from pediatric vaccines in high-income countries was a precautionary decision, rooted in the principle:
“When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause-and-effect relationships are not fully established scientifically.”
However, this principle has not been equally applied in developing countries, where cost, availability, and logistics still justify the use of multi-dose vials with thimerosal, raising ethical concerns about global vaccine equity.
