• An advisory panel at the CDC has passed a vote 5–1 in favor of recommending flu vaccines that do not use the preservative thimerosal.

  • Thimerosal is a mercury-based compound found in some vaccines and pharmaceutical products.

  • Healthcare professionals and medical advocacy organizations are concerned that this recommendation might cause increased public hesitancy towards vaccines.

A 5-1 vote by a CDC advisory committee on Thursday has led to the recommendation of only using thimerosal-free, single-dose flu vaccines.

This voting outcome comes after a significant restructuring of the Advisory Committee on Immunization Practices (ACIP), initiated by Health Secretary Robert F. Kennedy Jr., who replaced all 17 members with his selections.

Medical professionals and organizations like the American Medical Association (AMA) and the American Academy of Pediatrics (AAP) express concerns that these actions may promote confusion and distrust toward vaccine safety.

“This will only serve to erode confidence in vaccines further, despite overwhelming evidence that proves thimerosal’s efficacy and safety as a preservative,” explained Jake Scott, MD, a clinical associate professor of infectious diseases at Stanford Medicine.

Thimerosal, a compound that includes mercury, has been utilized since the 1930s in various vaccines and medications. Since the 1990s, it has faced scrutiny and controversy, particularly with claims—now proven false—that it was linked to autism and neurotoxic effects.

The new guidelines by ACIP contradict extensive research and many years of data indicating no connection between thimerosal and autism or other neurological issues.

“The decision seems to be devoid of any data-driven justification,” commented Saahir Khan, MD, PhD, from Keck Medicine of USC. “It is likely to foster mistrust and possibly reduce vaccination rates by indirectly suggesting that thimerosal-containing vaccines are unsafe, contrary to what evidence has shown.”

Understanding Thimerosal

Thimerosal, which is approximately 50% mercury by weight, is commonly used in vaccine preservation.

Each time a drug vial is accessed, it risks contamination from bacteria and fungi, which thimerosal helps prevent.

With the rise of single-dose vaccines, the prevalence of thimerosal in vaccines has decreased.

For children under the age of six in the U.S., all routinely recommended vaccines are available without thimerosal. Last flu season, approximately 96% of flu vaccines administered were thimerosal-free.

The recent ACIP recommendation against thimerosal in flu vaccines specifically targets those contained in multi-dose vials, although these vials constitute only a minor fraction of the flu vaccines available. However, they are crucial from a public health perspective.

“Multi-dose vials are more cost-effective, easier to distribute, and vital for maintaining equitable access, especially during pandemics where single-dose supplies may quickly deplete,” explained Scott.

In 1999, following a review prompted by the FDA Modernization Act (FDAMA), U.S. health authorities including the FDA and CDC recommended removing thimerosal from childhood vaccines as a precaution, despite no evidence of harm other than minor local reactions. This action was aimed at reducing overall mercury exposure in infants, some of whom were receiving mercury quantities exceeding EPA guidelines in their first six months.

In vaccine doses, thimerosal contributes about 25 micrograms of mercury per 0.5 mL dose, equivalent to what is found in a three-ounce can of tuna fish.

However, it’s critical to distinguish between the mercury in thimerosal and that in fish. Fish contain methylmercury, which is toxic, whereas thimerosal contains ethylmercury, which is expelled from the body much more quickly and is less likely to accumulate and cause harm.

Reexamining the Debate Over Thimerosal

In anticipation of this week’s ACIP vote, Lyn Redwood, a retired nurse practitioner and president emeritus of Children’s Health Defense, reiterated numerous concerns about thimerosal during a CDC presentation.

Despite her claims, particularly regarding a link between thimerosal and neurological issues, extensive research including controlled studies, meta-analyses, and trials, has consistently shown no connection between thimerosal in vaccines and any neurological conditions.

An often referenced study from Denmark, involving over half a million children, showed no link between thimerosal and autism. Similarly, a large UK study involving over 100,000 children also found no connection to neurodevelopmental disorders.

A 2010 study further supported these findings, showing no increased risk for autism spectrum disorder (ASD) from exposure to thimerosal during prenatal or infant stages.

Significantly, even after the removal of thimerosal from vaccines, autism rates have continued to rise, as observed in Denmark since 1991 and in the U.S. since 2003, highlighting that autism rates are not influenced by the presence of thimerosal in vaccines.

Medical professionals stress that ignoring the substantial evidence supporting vaccine safety could have serious health implications for the population.

“Based on solid research and my clinical experience, I advocate for vaccinations. Unfortunately, some individuals will choose not to vaccinate, increasing their own risk of infection and endangering others around them,” Khan noted.