Hidden flaws in vaccine safety studies: Unpacking the truth about thimerosal, vaccines and autism

03/05/2025 / By Olivia Cook

Several major studies cited by health authorities claim thimerosal in vaccines is safe, but they contain serious methodological flaws, including biased data selection, inconsistent diagnostic criteria and statistical techniques that obscure potential risk.
Five of the six studies reviewed were directly commissioned by the CDC, raising concerns about bias. Some findings indicating a potential link between thimerosal and neurodevelopmental disorders were omitted or downplayed in published reports.
Studies often failed to account for the time lag between birth and autism diagnosis, leading to misleading conclusions. In some cases, children were included in control groups before they were old enough for an autism diagnosis. Additionally, overmatching – where cases and controls were too similar – masked true differences in thimerosal exposure levels.
While CDC-backed studies report no link between thimerosal/thimerosal-containing vaccines and autism, independent research spanning over 75 years has consistently found associations between thimerosal exposure and neurodevelopmental disorders, including attention deficits, speech delay and tics.
Given the significant methodological concerns and conflicts of interest, more independent, transparent research is necessary to fully assess the safety of thimerosal in vaccines, ensuring public trust in vaccine safety data.

For decades, the use of thimerosal – a mercury-based preservative in some vaccines – has sparked intense debate among scientists, doctors and the public. While the Centers for Disease Control and Prevention (CDC) and other public health agencies maintain that thimerosal is “safe,” independent research has raised concerns about its potential risks, particularly in connection to neurodevelopmental disorders, including autism.

This article explores the controversy, breaking down the science behind thimerosal and the conflicting studies surrounding its safety, as reported in the review article “Methodological issues and evidence of malfeasance in research purporting to show thimerosal in vaccines is safe,” published in the journal BioMed Research International.

While thimerosal has been removed from most childhood vaccines in the U.S. since 2001, it is still present in some flu shots and vaccines used in other countries. Over 165 studies have examined its effects, with many identifying potential links to a range of health issues including:

Neurodevelopmental disorders – Autism, attention deficit disorder/attention deficit hyperactivity disorder (ADD/ADHD), speech and language delays and tics (repetitive movement or sounds) are among the disorders associated with thimerosal exposure.
Autoimmune conditions – These include disorders where the immune system attacks the body.
Allergic reactions – Children have experienced severe allergic responses and physical abnormalities.
Toxicity concerns – Some studies have even reported fatalities and cases of poisoning following exposure to thimerosal.

Despite this, the CDC insists there is “no connection between thimerosal and autism,” basing its stance on several key studies. However, these studies have been criticized for methodological flaws and potential bias.

The Madsen et al. (2003) study: Data manipulation?

Analyzing data from 1971 to 2000, this Danish study claimed that autism rates increased after thimerosal was removed from vaccines in 1992. However, several concerns have been raised, including:

Data criteria changed. The shift from ICD-8 to ICD-10 diagnostic standards in 1994 led to a significant increase in diagnosed cases, which are unrelated to thimerosal removal.
Data exclusion. The study omitted 2001 data, which showed a decline in autism rates after thimerosal was phased out – raising concerns about cherry-picked results.
Clinic inclusion bias. A major autism center in Copenhagen was not included in the pre-1993 data but was later added, likely inflating post-thimerosal autism rates.

The Stehr-Green et al. (2003) study: Incomparable data?

This study compared autism rates in California, Denmark and Sweden against the thimerosal exposure levels. It found no clear (or consistent) link between thimerosal and autism but the study had key flaws.

Data reliability issues. Swedish autism data only accounted for hospital visits, potentially missing many cases diagnosed in outpatient settings. The California data included a broader range of autism spectrum disorders, which could have skewed the results.
Inconsistent thimerosal exposure. The three countries had vastly different vaccination policies and thimerosal exposure levels, making direct comparisons unreliable.

The Hviid et al. (2003) study: Counting problems?

This population-based Danish cohort study examined over 500,000 children who received thimerosal-free vaccines and those who received thimerosal-containing vaccine and found no link between thimerosal and autism. However, concerns include:

Age inclusion bias. The study included children as young as one-year-old, even though autism is typically diagnosed after age four. This could have led to an underestimation of autism cases.
Flawed counting method. Researchers counted “person-years of follow-up” instead of actual autism cases, which could have diluted the apparent impact of thimerosal exposure.

The Andrews et al. (2004) study: Data integrity issues?

The UK-based study used medical records to examine whether thimerosal exposure was associated with developmental disorders. It found that higher exposure was linked to fewer general developmental issues but more cases of tics. Concerns include:

Data errors. The study used a database known to have significant inaccuracies, which could have affected the results.
Statistical manipulations. The use of a non-transparent regression technique made it difficult to assess the true impact of thimerosal on autism risk.
Different exposure levels. The U.K.’s vaccine schedule was different from that of the U.S., making the findings less applicable to American children.

The Versatraeten et al. (2003) study: Changing conclusions?

Analyzing data from several health maintenance organizations (HMOs), this CDC study initially found a 7.6-fold increased risk of autism with thimerosal exposure but later concluded there was no significant relationship. Issues raised include:

Multiple revisions. The study underwent several phases of methodological changes, progressively weakening the original findings.
Potential bias. Internal emails obtained through the Freedom of Information Act suggest the lead investigator may have faced pressure to alter the results to downplay the connection between thimerosal and autism.

The Price et al. (2010) study: Overmatching concerns?

The case-control study analyzed data from three healthcare organizations and found no significant link between prenatal or infant thimerosal exposure and autism. However, criticisms of the study include:

Overmatching issues. The study matched cases and controls of children with similar characteristics, including the same vaccination schedules, which could have masked the true impact of thimerosal on autism risk.
Data suppression. Unpublished internal CDC reports showed significant findings linking prenatal thimerosal exposure to regressive autism, raising concerns about transparency and the study’s integrity.

Vaccines are vital to protecting public health BUT ensuring their safety should be an ongoing process. The thimerosal controversy and debate highlights the need for rigorous, unbiased research and open scientific dialogue.

The controversy over thimerosal is far from settled. While the CDC maintains that thimerosal is safe, independent research has raised valid concerns that cannot be ignored. As new research emerges, health organizations must re-evaluate their stance and ensure that public health policies are based on the most reliable and unbiased evidence.

Thimerosal has been largely removed from childhood vaccines in the U.S. but it remains in some flu shots and vaccines used globally. Parents and healthcare providers should stay informed and base their decisions on the most transparent, up-to-date research. Public trust in vaccines depends on open, honest and science-driven discussions about their safety.

Watch this video about thimerosal, which is dangerous at any concentration.

This video is from the Vaccines and Vaccinations channel on Brighteon.com.