The Thimerosal Controversy in Vaccines

A century ago, concerns about vaccine safety often revolved around bacterial contamination. A tragic incident occurred in 1916 when four children died in South Carolina after receiving a typhoid vaccine contaminated with Staphylococcus aureus. Similarly, in 1928, twelve children in Queensland, Australia, died from diphtheria immunizations that were tainted.

In the 1930s, vaccine manufacturers started using a preservative called thimerosal to prevent microbial growth. This went largely unnoticed for about sixty years, with the main side effects being minor reactions at the injection site, as reported by the US FDA.

This changed in 1999 when US health officials urged drug companies to eliminate thimerosal from vaccines. Although there was no proof that thimerosal was harmful in the amounts used, it contains a form of mercury, raising concerns about its potential neurotoxicity in childhood vaccinations.

Since then, most vaccines have had thimerosal removed, except for a few, such as some multidose flu vaccines. Multiple studies have now established that there’s no link between thimerosal in vaccines and neurodevelopmental issues like autism, even as autism rates continue to rise.

Thus, it was puzzling for public health experts when the preservative was included in the agenda for a recent meeting of outside vaccine advisers with the US Centers for Disease Control and Prevention (CDC).

Dr. Sean O’Leary, a pediatrician at Children’s Hospital Colorado and former liaison to the CDC’s Advisory Committee on Immunization Practices, noted he’s not aware of pediatric practices using multidose flu vaccines.

This meeting marked the first convening since US Health and Human Services Secretary Robert F. Kennedy Jr. dismissed the committee’s previous members, citing conflicts of interest, then appointed eight new advisors. Many experts and lawmakers have questioned the qualifications of these new panelists. In fact, one member withdrew just before the meeting began due to a financial review.

A presentation on thimerosal in vaccines is scheduled for Thursday morning, along with proposed recommendations concerning its use in flu vaccines. The meeting will wrap up with a vote that could potentially influence CDC policy—though without a confirmed director, it may fall to Kennedy.

The sudden inclusion of thimerosal on the agenda raised alarms among vaccine experts who see the science as settled, fearing that Kennedy, known for leading an anti-vaccine group called Children’s Health Defense, could be trying to instill doubts about vaccine safety. Another former leader from the same group, Lyn Redwood, is expected to present at the meeting.

So, how did thimerosal become such a contentious topic? In 1997, Rep. Frank Pallone, addressing mercury pollution concerns, advocated for federal requirements to catalog mercury compounds in drugs and food. This led to a shocking realization: some infants could have received amounts of thimerosal exceeding what was considered safe based on existing guidelines.

It’s important to note that the limits were established for methylmercury, which is toxic and found in some fish. Thimerosal, however, contains ethylmercury, a compound that is less stable and clears from the body more rapidly, thus being less likely to cause harm.

The discovery triggered numerous emergency meetings among health officials to discuss potential actions. Despite no evidence indicating harm from thimerosal, the prevailing sentiment was to eliminate it, as vaccines were moving to single-dose containers that wouldn’t need preservatives, even if it raised production costs.

Despite all this, the removal process sped up in 1999, leading to thimerosal being eliminated or reduced in most vaccines recommended for children under six by 2001. The public health message was clear: they aimed to make safe vaccines even safer.

However, the damage was done. Vaccines had already been controversially linked to autism back in 1998 by British doctor Andrew Wakefield, who published a now-retracted study falsely claiming a connection between the MMR vaccine and autism, even though the MMR vaccine never contained thimerosal.

Parents of children with autism were desperate for answers, including Redwood, who argued that her son received far more mercury than was deemed acceptable by EPA guidelines. She later became an advocate, discussing thimerosal and autism at various conferences.

This uproar over the potential link between thimerosal and autism was unexpected for Orenstein and his colleagues. As he pointed out, mercury had never been substantially linked to autism, but it still became a significant concern.

This led to numerous studies, including a 2004 analysis by the Institute of Medicine that found the evidence did not support a causal relationship between thimerosal-containing vaccines and autism. The same conclusion was drawn for MMR vaccines. A 2010 CDC study also found no link. In fact, after thimerosal was largely removed, autism rates continued to rise, contradicting the expectations.

Yet, voices calling for attention to disproven links linger, particularly from Kennedy, who published a book in 2014 demanding immediate thimerosal removal, claiming it posed risks to millions of children.

Thimerosal still appears in a small percentage of flu vaccines, primarily in multidose vials. The Vaccine Integrity Project suspects that its appearance on the ACIP agenda is to fuel public discussion about vaccine risks.

HHS hasn’t responded to inquiries regarding Redwood’s participation, and Redwood herself declined to comment. Nonetheless, her presentation slides ahead of the meeting maintained that thimerosal is a safety concern. Some initial claims in her slides cited studies that don’t appear to exist, leading to their removal due to discrepancies in findings.

The CDC recently offered a comprehensive review of studies asserting that there’s no connection between thimerosal-containing vaccines and autism.

During a recent Senate confirmation hearing, Dr. Susan Monarez, the candidate for CDC leadership, stated, “I have not seen a causal link between vaccines and autism.”

Interestingly, the decision to phase out thimerosal also had unintended consequences. At the time, health authorities advised delaying hepatitis B vaccinations for certain infants, and this led some hospitals to stop administering the shot altogether, resulting in the tragic death of a child who was born to a mother infected with the virus.

Reflecting on the thimerosal phase-out, Orenstein acknowledged it was a difficult choice. His concern was that retaining it could lead to harm if studies later proved it was hazardous. However, he clarified that autism was not a factor in the initial decision.