Twenty-five years ago, the Advisory Committee on Immunization Practices (ACIP) to the CDC (Centers for Disease Control) was considering what to do about thimerosal, an ethylmercury-containing preservative contained in several vaccines. Thimerosal was used in multi-dose vials to prevent contamination with bacteria. When the rubber stopper is pierced again and again to withdraw each dose, bacteria might be inadvertently injected into the vial, putting recipients of the eighth, ninth, or tenth doses at risk.

Before the use of preservatives, children injected with contaminated multi-dose vaccines developed cellulitis, abscesses, and bloodstream infections. For that reason, thimerosal, which eliminated the problem, has been used as a preservative in multi-dose vials since the 1930s.

On November 21, 1997, an amendment to the Food and Drug Administration (FDA) Modernization Act was signed into law. The amendment gave the FDA two years to “compile a list of drugs and food that contain intentionally introduced mercury compounds and … [to] provide a quantitative and qualitative analysis of the mercury compounds on the list.”

The impetus behind the amendment was that mercury at high levels was known to be toxic to the brain. In 1997, thimerosal was contained in the DTaP, hepatitis B, and Haemophilus influenzae type b (Hib) vaccines. Anti-vaccine groups pounced, claiming that thimerosal was causing brain damage, including autism. No one more so than Robert F. Kennedy Jr., who wrote a book titled, Thimerosal: Let the Science Speak (in which the science never actually spoke).

Perhaps the most ironic outcome … Even though RFK Jr. promoted the notion that thimerosal in vaccines was causing autism, since its removal, the incidence of autism has increased from one in 150 children in 2000 to one in 32 children today.

In 1999, when the ACIP considered this issue, the facts about ethylmercury were largely reassuring:

• First, thimerosal was a trivial addition to the burden of mercury encountered from living on the planet Earth. Because mercury is in the earth’s crust, we ingest mercury every day. It’s in air, soil, and water or anything made from water, such as breast milk and infant formula. Environmental mercury is methylmercury, which has a much longer half-life than the ethylmercury in some vaccines, so it is more likely to accumulate. (Think of the relative differences between ethylalcohol — contained in beer and wine — and methylalcohol, which is added to gasoline and antifreeze and is poisonous).

Because the burden of environmental mercury is much greater than mercury contained in vaccines, studies have shown that the level of mercury in the bloodstream doesn’t change after receiving thimerosal-containing vaccines. In other words, the mercury contained in vaccines was an undetectable addition to the burden of mercury ingested from living on Earth.

• Second, studies of thimerosal showed that it wasn’t harmful. Western Europe had removed thimerosal-containing vaccines in the early 1990s. Also, some Canadian provinces used thimerosal-containing vaccines and others used the same vaccines that didn’t contain thimerosal. This made it easy to perform studies. About 10 studies have shown that children who had received thimerosal containing vaccines were not at greater risk of neurodevelopmental problems, including autism, than those who received the same vaccines that didn’t contain thimerosal.

Despite this reassuring evidence from 25 years ago, both the CDC and the American Academy of Pediatrics (AAP) supported the removal of thimerosal from vaccines, which basically eliminated multi-dose vaccine vials for children younger than six years of age. A statement by the AAP in July 1999 revealed the inexplicable nature of the decision. “The current levels of thimerosal will not hurt children,” they wrote. “But reducing those levels will make safe vaccines even safer.” If thimerosal in vaccines was safe, then taking it out didn’t make vaccines safer. It only made it perceived to be safer, which isn’t the same thing.

Just a warmup for RFK

The fallout was immediate; certain vaccines became less available and more expensive. A 3-month-old child in Michigan died of hepatitis B infection because the hospital where the child was born had suspended the hepatitis B vaccine for fear of thimerosal. Developing countries were hardest hit by the price increase and challenging storage issues precipitated by moving from multi-dose vials to single-dose vials.

Anyone watching these events from a distance could reasonably conclude that the CDC wouldn’t have eliminated thimerosal as a preservative in vaccines for young children unless it was dangerous, even though it wasn’t dangerous. Indeed, two anti-vaccine groups, Generation Rescue and Moms Against Mercury, were born of the incident. It was an exercise in how not to communicate theoretical risks.

Perhaps the most ironic outcome of the event was that, even though Robert F. Kennedy Jr. had promoted the notion that thimerosal in vaccines was causing autism, since its removal, the incidence of autism has increased from one in 150 children in 2000 to one in 32 children today.

Now, by attacking aluminum adjuvants in vaccines, RFK Jr. can make himself and his personal injury lawyer friends rich again — all at the expense of children’s health.

On June 26, 2025, Lyn Redwood, an anti-vaccine activist who had lobbied the CDC to remove thimerosal-containing vaccines 25 years ago, made a presentation to RFK Jr.’s handpicked, science-averse, expertise-light new ACIP. Redwood’s presentation was full of misrepresentations and inaccuracies. Normally, presentations before an ACIP vote are reviewed by subject matter experts. Not this time. Redwood was free to make bogus claims and mislead the committee, who then voted to eliminate thimerosal-containing vaccines for adults.

The impact of this vote on influenza vaccine availability and cost has yet to be determined. As was true 25 years ago, this decision will not make the influenza vaccine safer. It will only cause people to fear vaccines unnecessarily.

The anti-science thimerosal vote at the ACIP was just a warm-up for RFK Jr.’s real agenda. Many believe that RFK Jr.’s next target will be aluminum salts, which are used as adjuvants that enhance the immune response, allowing for fewer doses and lesser quantities of the vaccine. Aluminum adjuvants are contained in seven vaccines currently given to children. Presumably, RFK Jr. will follow the same playbook. Enlist an anti-vaccine activist (like Lyn Redwood) to present flawed data to the committee, then ask the committee to vote on eliminating aluminum-based adjuvants from vaccines. Unlike thimerosal, which simply resulted in switching from multi-dose vials to single-dose vials, aluminum can’t be removed from vaccines without rendering them ineffective.

But that’s not RFK Jr.’s real goal. Given his lengthy history of suing vaccine makers for false claims of harm, what he really wants is to add aluminum adjuvants to the list of compensable injuries in the Vaccine Injury Compensation Program (VICP). Or to remove those vaccines from the VICP and expose them to civil litigation in open court. Then we will be right back to where we were in the early 1980s when, before the VICP was put into effect, we went from 18 vaccine makers in 1980 to four by the end of the decade — all based on false claims of harm caused by the pertussis vaccine. The pertussis vaccine litigation made personal-injury lawyers rich.

Now, by attacking aluminum adjuvants in vaccines, RFK Jr. can make himself and his personal-injury lawyer friends rich again — all at the expense of children’s health. “The only thing that we learn from history,” wrote Georg Wilhelm Hegel, “is that we learn nothing from history.”

Paul A. Offit, MD, is director of the Vaccine Education Center and professor of pediatrics in the Division of Infectious Diseases at Children’s Hospital of Philadelphia. This piece originally ran on his Substack, Beyond the Noise!

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