Is Thimerosal in Flu Vaccines Dangerous or Safe?

Thimerosal, a mercury-based preservative used in some flu shots, has returned to the center of controversy after a dramatic policy shift by a newly reshaped CDC vaccine advisory panel. On June 26, 2025, the panel voted to restrict thimerosal-containing influenza vaccines, raising questions not only about science and public health but also about legal authority, transparency, and global implications.

What Is Thimerosal?

Thimerosal is a preservative containing ethylmercury. It has been used in vaccines since the 1930s to prevent contamination in multi-dose vials. Its rapid breakdown in the body and short half-life distinguish it from methylmercury, the neurotoxin found in fish.

Scientific Consensus: Thimerosal Is Safe

Extensive research by organizations such as the CDC, WHO, and NIH confirms thimerosal’s safety. Multiple studies across different populations have found no evidence linking thimerosal to autism or other neurological conditions. Ethylmercury clears the body much faster than methylmercury and does not accumulate.

For perspective: a flu shot contains 25 micrograms of ethylmercury, less than a small can of tuna fish.

The CDC Vote: What Changed?

The Advisory Committee on Immunization Practices (ACIP) was restructured in June 2025 by U.S. Health Secretary Robert F. Kennedy Jr., a long-time vaccine skeptic. All 17 previous members were dismissed and replaced by new appointees, some lacking formal credentials or publications.

In a 5-1 vote (with one abstention), the panel recommended against thimerosal in all flu vaccines. A planned CDC presentation summarizing decades of safety data was withdrawn before the meeting.

Legal Analysis: Did the Panel Overstep?

1. Authority of the Secretary of Health

Under U.S. administrative law, the Health and Human Services Secretary has broad authority to appoint advisory panel members. However, dismissing the entire ACIP en masse raises questions about political interference in public health policymaking.

While the appointments are legally permissible, critics argue that such actions may violate norms of scientific independence, especially when driven by ideology rather than evidence.

2. Due Process and Transparency

The removal of CDC-authored presentations, along with the use of unverified data, may violate federal transparency and data quality guidelines, including:

Information Quality Act (IQA)
Federal Advisory Committee Act (FACA)
OMB Guidelines on Scientific Integrity

If litigation arises, courts may review whether the panel’s recommendations were “arbitrary or capricious” under the Administrative Procedure Act (APA).

3. Conflict of Interest & Accountability

As of the panel vote, new members had not filed standard conflict of interest disclosures. This failure undermines the committee’s credibility and could form the basis of legal challenges or Freedom of Information Act (FOIA) investigations.

Global Impact: Vaccine Access at Risk

Many countries rely on multi-dose vials containing thimerosal because they are cheaper, easier to store, and more efficient during mass immunization campaigns. If international agencies follow the U.S. lead, it could:

Increase vaccine production costs
Disrupt immunization programs
Reduce access in low-resource settings

As Dr. Cody Meissner noted, such a move may cause more harm than any hypothetical risk from thimerosal.

Misinformation Risk: Ideology Over Evidence?

Thimerosal has long been targeted by anti-vaccine activists, despite overwhelming scientific consensus about its safety. The ACIP vote, led by a panel aligned with anti-vaccine rhetoric, may inadvertently validate pseudoscientific claims and deepen public confusion.

Replacing peer-reviewed evidence with unvetted presentations—one of which cited a fabricated study—poses serious challenges to evidence-based policymaking.

Ethylmercury vs Methylmercury: Not the Same

Property	Ethylmercury (Thimerosal)	Methylmercury (Seafood)
Toxicity	Low	High
Brain accumulation	No	Yes
Blood half-life	7 days	44–58 days
Found in vaccines?	Yes	No

This difference is key to understanding why the fear around thimerosal is misplaced.

Policy Consequences

In the U.S.:

Clinicians may hesitate to use thimerosal-containing vaccines.
Manufacturers may preemptively stop producing them for U.S. markets.
Public trust may be further eroded.

Globally:

WHO and UNICEF may reevaluate procurement policies.
Supply chains may be disrupted.
Costs could rise for low-income countries.

Conclusion

The June 2025 CDC panel vote on thimerosal marks a sharp break from decades of science-based public health policy. While framed as a safety measure, the decision reflects broader ideological trends that may impact vaccine availability and trust—not just in the U.S., but worldwide.

It’s critical for journalists, scientists, and legal professionals to ensure accountability, transparency, and scientific integrity remain at the core of vaccine policymaking.