The topic of <a href="https://news.quantosei.com/2025/06/28/rfk-jrs-vaccine-panel-recommends-new-rsv-treatment-for-infants/” title=”Breakthrough Infant RSV Shot Backed by CDC Panel”>thimerosal, a preservative used in certain vaccines, has recently re-entered public discussion, particularly in relation to seasonal flu shots. This resurgence stems from a recommendation made on June 26, 2025, by vaccine advisers under the Trump administration. Despite decades of scientific consensus affirming its safety, the panel advised that Americans should seek out flu vaccinations that do not contain this ingredient.
This renewed focus on thimerosal has puzzled many medical professionals. Extensive scientific studies have consistently shown that the low doses used in vaccines pose no risk of harm. Experts worry that revisiting this settled issue could undermine public confidence in safe and effective vaccines.
Understanding Thimerosal: Purpose and History
Thimerosal is an ethylmercury–based preservative. It has been used in some vaccines since the 1930s. Its primary role is to prevent dangerous bacterial and fungal contamination. This is especially important in multi-dose vials. A multi-dose vial is accessed multiple times. Each needle puncture carries a small risk of introducing microbes. The preservative stops these microbes from growing. This ensures the vaccine remains safe for everyone receiving a dose from that vial.
The concern about thimerosal first emerged in the late 1990s. This was due to its mercury content. However, it’s crucial to distinguish between ethylmercury and methylmercury. Methylmercury is a different type of mercury. It is found in some seafood. High levels of methylmercury can be toxic and accumulate in the body. Ethylmercury, conversely, is processed differently by the human body. It is broken down and excreted relatively quickly.
Scientific Consensus on Thimerosal Safety
Numerous scientific studies have thoroughly investigated the safety of thimerosal in vaccines. These studies have consistently found no evidence of harm. They have specifically looked for links between thimerosal and neurodevelopmental disorders, including autism. A Centers for Disease Control and Prevention (CDC) analysis prepared for the recent advisory meeting explicitly reaffirmed these findings.
The amount of ethylmercury per vaccine dose is very small. Studies show vaccinating infants with thimerosal does not raise blood mercury levels above safe values. This is because ethylmercury is rapidly eliminated from the blood. It is primarily excreted via stools. Experts like Dr. Sean O’Leary of the American Academy of Pediatrics have stated that the continued focus on thimerosal is a “nonissue.” They emphasize the strong body of research confirming its safety.
In fact, some experts point out a key piece of counter-evidence regarding the autism link. Autism rates in the U.S. continued to rise after thimerosal was removed from most childhood vaccines starting in 2001. This contradicts the hypothesis that thimerosal causes autism. Leading health organizations like the CDC and the Food and Drug Administration (FDA) maintain there is no evidence that low doses of thimerosal in vaccines cause harm.
Thimerosal’s Phased Removal and Current Use
As a precautionary measure, the U.S. public health agencies began phasing thimerosal out of childhood vaccines. This decision was made in the early 2000s. It was not based on scientific evidence of harm. It was done out of an abundance of caution.
Since 2001, all vaccines routinely recommended for children aged 6 and younger in the U.S. come in formulations that do not contain thimerosal. Many common childhood vaccines never contained thimerosal at all. This includes vaccines for chickenpox, polio, and measles, mumps, and rubella (MMR).
Today, thimerosal’s use in U.S. vaccines is very limited. It is primarily found in a small subset of flu shot formulas. These are mainly in multi-dose vials. These vials can be used for adults or older children. The vast majority of flu vaccines administered in the U.S. are already thimerosal-free. According to CDC data from the previous fall and winter season, 96% of all flu vaccines given in the U.S. were thimerosal-free. This percentage was even higher in federal programs like Vaccines for Children. Most children typically receive their flu vaccination from a single-dose shot, which is also usually thimerosal-free. Only about 4% of the U.S. flu vaccine supply currently comes in multi-dose vials containing thimerosal.
The Reopened Debate and Concerns
The recent revival of the thimerosal debate is closely tied to U.S. Health Secretary Robert F. Kennedy Jr. Kennedy has a long history of promoting false claims linking thimerosal to autism. He has been a prominent figure in antivaccine activism for two decades. The new vaccine advisory committee, appointed by Kennedy, includes members who have expressed skepticism about vaccines.
During the June 26, 2025 meeting, the panel reportedly heard a presentation from Lyn Redwood. Redwood is a nurse practitioner. She previously ran an anti-vaccine group founded by Kennedy. She argued for removing thimerosal from remaining flu shots. Among her claims were potential risks to pregnant women.
However, the scientific evidence does not support these claims. A CDC analysis prepared for the meeting, though reportedly not allowed full public presentation, found no link between thimerosal and autism or other neurodevelopmental disorders. Furthermore, studies have found no scientific evidence of adverse events among children born to women who received flu vaccine containing thimerosal during pregnancy.
Experts have criticized the handling of the meeting. They note that the committee did not follow its usual evidence-based procedures. Panelist Dr. Joseph Hibbeln reportedly acknowledged the lack of “demonstrable evidence of harm” from thimerosal. He stated the decision wasn’t strictly based on evidence. Instead, it was to “respect the fear of mercury.”
Public health professionals voiced sharp concerns. Dr. Jason Goldman of the American College of Physicians described the meeting as potentially “orchestrated to undermine the well-established safety and efficacy of vaccines.” Critics worry that this discussion, despite the overwhelming scientific data, will fuel misinformation. This could potentially increase vaccine hesitancy. This comes at a time when vaccine uptake is already a concern.
Impact of the Recommendation
The panel’s recommendation to use only thimerosal-free flu shots aligns with the current reality in the U.S. Most flu shots administered are already free of the preservative. Therefore, the practical impact on vaccine availability for most people is expected to be minimal. There are numerous thimerosal-free options readily available, including most single-dose shots and the nasal spray vaccine.
However, experts worry about the message this sends. Focusing on thimerosal, an ingredient proven safe and already largely removed, provides unnecessary ammunition for anti-vaccine groups. It distracts from the importance of vaccination itself. It also ignores the robust scientific consensus that thimerosal in vaccines is safe. While phased out in the U.S. primarily as a precaution, thimerosal is still safely used in vaccines in many other countries worldwide.
The scientific community remains united. The extensive body of research confirms the safety of thimerosal at the doses used in vaccines. Promoting vaccine confidence requires clear communication based on this established science, not on unfounded fears from a debate that has been settled by evidence for years.
Frequently Asked Questions
What is thimerosal and why is it used in some vaccines?
Thimerosal is a mercury-based compound. It is used as a preservative in some vaccines. Its main purpose is to prevent contamination by bacteria or fungi. This is crucial in multi-dose vaccine vials. These vials are punctured multiple times for different patients. Thimerosal prevents harmful microbes from growing inside the vial after it’s opened. This ensures each dose from the vial is safe.
Is thimerosal in flu shots safe, according to science?
Yes, according to extensive scientific research and major health organizations like the CDC and FDA, thimerosal in vaccines is safe. Studies have found no evidence linking the ethylmercury in thimerosal to autism or other neurodevelopmental disorders at the low doses used in vaccines. Ethylmercury is processed and excreted by the body much faster than methylmercury, the toxic form found in some fish.
Which vaccines currently contain thimerosal in the U.S.?
Today, thimerosal is found in a very small percentage of vaccines in the United States. All routine vaccines recommended for children aged 6 and younger have been available in thimerosal-free formulations since 2001. Thimerosal is primarily present only in about 4% of the U.S. flu vaccine supply, specifically in multi-dose vials. Most flu shots administered, including nearly all single-dose shots, are already thimerosal-free.
Conclusion
While a recent recommendation has brought the topic back into the spotlight, the scientific consensus on thimerosal remains clear: it is a safe and effective preservative used in a dwindling number of vaccines, primarily multi-dose flu shots. The vast majority of vaccines administered in the U.S., including most flu vaccines, are already thimerosal-free. The decades of research consistently show no link between thimerosal at vaccine doses and adverse health effects, including autism. Public health relies on trust built on evidence, and the established science strongly supports the safety profile of vaccines, whether they contain thimerosal or not. Focusing on unfounded fears risks undermining confidence in crucial public health tools like vaccination.
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