CDC Report Finds No Connection Between Thimerosal and Autism
Thimerosal is a mercury-containing compound that has been widely used as a preservative in some biological and drug products since the 1930s.
In a new CDC report, investigators found that there was no connection between thimerosal in vaccines and autism or other neurodevelopmental conditions. Thimerosal is a mercury-containing compound that has been widely used as a preservative in some biological and drug products since the 1930s. The properties of thimerosal were documented to contribute to the safe use of multidose vials and packaging of vaccines, such as seasonal and pandemic influenza vaccines.1,2
Thimerosal is a mercury-containing compound that has been widely used as a preservative in some biological and drug products since the 1930s. | Image Credit: Mikhailov Studio - stock.adobe.com
Prior to the introduction of thimerosal in vaccines, there had been data available for several animal species as well as humans, which provided evidence that it was safe and effective as a preservative. Since then, it has been the subject of many studies, especially for its effect on neurodevelopmental outcomes.
Preservatives are used to kill or prevent the growth of bacteria and fungi, and though the risk of contamination cannot be completely eliminated, it does help to prevent microbial growth in the instance that a vaccine is accidentally contaminated, especially with the use of multidose vials.2
Currently, there are 3 FDA-approved seasonal influenza vaccines that contain a concentration of mercury, as evident from January 15, 2025, on the CDC website. These include Afluria (multidose presentation), Flucelvax (multidose presentation), and Fluzone (multidose presentation), all with a 0.01% concentration of thimerosal. The use of the preservative used in FDA-licensed vaccines has significantly declined and were developed into single-dose containers instead of multidose. For children ages 6 years and younger, all vaccines routinely recommended do not contain thimerosal.2
During the 2024-2025 flu season, approximately 94% of all influenza vaccines in the United States were thimerosal-free, and 98% of federal vaccines through Vaccines for Children or the 317 program were also thimerosal-free, according to the CDC report. Further, in an analysis of flu vaccines in 2024, only 0.3% of doses administered to pregnant women contained thimerosal.1
In 5 studies that were part of the CDC report, prenatal exposure to thimerosal-containing vaccines was not associated with autism spectrum disorder (ASD). In one study, the prenatal exposure to the Pandemrix H1N1 vaccine was not associated with ASD between October 2009 and September 2010 in Sweden. In another large cohort study in Denmark, children exposed prenatally to the vaccine were not more likely to experience early childhood morbidity, including ASD diagnoses, compared with children not exposed to the vaccine.1,3,4
In 2 other studies, children with an autism diagnosis were evaluated for mercury exposure from vaccination since birth, and there were no increases of ASD observed that were associated with prenatal and early-life exposure to ethylmercury. For neuropsychological performance, there was no causal association between early exposure, such as prenatal or neonatal periods, to mercury from thimerosal-containing vaccines and immune globulins and deficits in neuropsychological functioning.1,5,6
In 14 studies, similar results were seen for the diphtheria-tetanus-pertussis, diphtheria-tetanus, hepatitis, and measles, mumps, and rubella vaccinations and antibody-stimulating proteins and polysaccharides in vaccines. Further, investigators found no association between thimerosal exposure and attention deficit hyperactivity disorder.1
However, there have been studies by non-CDC authors, using data from the Vaccine Adverse Event Reporting System and the Vaccine Safety Datalink, that showed an association between thimerosal-containing vaccines and neurodevelopmental disorders, but these studies had significant methodological limitations. One of the main concerns was the diagnostic criteria to ASD changed, but other concerns included reliance on aggregated data for analysis, inability to control confounding factors, substantial risk for misclassification of thimerosal exposure, etc.1
The CDC Advisory Committee on Immunization Practices will meet June 25, 2025, to discuss recommendations for COVID-19 and respiratory syncytial virus immunizations, and on June 26, 2025, to discuss influenza immunization, chikungunya vaccines, anthrax vaccines, and MMR vaccines. They will also discuss recommendations for thimerosal-containing influenza vaccines.7
READ MORE: Immunizations Resource Center
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