Throwing Spaghetti at the Wall

Posted by Katherine J. Wu

https://www.theatlantic.com/health/archive/2025/06/vaccines-advisory-committee-acip-thimerosal-autism/683317/?utm_source=feed

Updated at 9:34 a.m. on June 25, 2025

Vaccine experts in the United States have long considered the case on thimerosal closed. A chemical preservative that stamps out contamination in vaccine vials, thimerosal was removed from most U.S. shots more than two decades ago over worries that its mercury content could trigger developmental delays. But those concerns—as well as baseless claims that thimerosal causes autism—have been proved unfounded, many times over. “We took care of this 20 years ago,” Kathryn Edwards, a pediatrician and vaccine expert at Vanderbilt University, told me.

That’s not how anti-vaccine activists see the compound. Even the strongest data supporting thimerosal’s safety have not quelled the concerns of those who insist on the chemical’s harms. And now the CDC’s Advisory Committee on Immunization Practices, or ACIP, has signaled that thimerosal’s presence in vaccines should remain open for debate. The panel is scheduled on Thursday to discuss the compound, which is present in a minority of flu shots in low or trace amounts, and vote on how vaccines containing it should be used.

The panel that will meet this week is more skeptical of vaccines than any version in ACIP history. Earlier this month, Secretary Robert F. Kennedy Jr. abruptly dismissed all 17 existing members of the panel—among them, some of the nation’s foremost experts in vaccinology, infectious disease, pediatrics, and public health—and replaced them with eight new members who largely lack expertise in vaccines and, in several cases, have espoused anti-vaccine viewpoints. This new panel will hear a presentation on thimerosal not from a career vaccine scientist—as is usual ACIP practice—but from Lyn Redwood, one of the first vocal advocates of the false notion that thimerosal causes autism and the former president of Children’s Health Defense, the anti-vaccine organization that Kennedy chaired until 2023.

ACIP’s charter is to evaluate the data and guide the country’s approach to vaccines. By reopening the case on thimerosal, Kennedy’s handpicked committee has already chosen to entertain a classic anti-vaccine talking point. If the new ACIP’s vote further limits the use of vaccines containing the compound, it will also show, from the get-go, how willing it is to disregard evidence.

A multitude of studies, going back more than 20 years, have shown that thimerosal has no link to autism. Children who have received thimerosal-containing vaccines aren’t at higher risk of developing autism. Nor has removing the compound from much of the vaccine supply in multiple countries—including the U.S.—decreased autism rates. Instead, autism rates have gone up. (Experts who study autism attribute that rise largely to more awareness and more sensitive diagnostics; Kennedy, meanwhile, insists, without evidence, that the uptick is the work of an “environmental toxin” that “somebody made a profit” on.)

But around the turn of the millennium, experts felt pressured to remove thimerosal from vaccines, especially those targeted to young children. After studies had linked chronic exposure to high levels of mercury found in fish and whale blubber to developmental delays, scientists began to worry about the element’s effects on the young brain. The FDA kick-started a campaign to suss out the mercury content of the products it oversaw. By 1999, researchers had pinpointed thimerosal as suspect: The levels of the type of mercury found in vaccines containing the compound seemed, at the time, worryingly high, Walter Orenstein, who directed the U.S.’s National Immunization Program from 1988 through 2004, told me. “So there were concerns that it might be harmful to children.” (Autism, notably, wasn’t a consideration.)

No research proved that harm, but the fears seemed theoretically legitimate. “It put us in a very difficult position,” Orenstein said. The studies necessary to thoroughly test whether the thimerosal in vaccines was toxic could take years; in the meantime, kids could suffer unnecessarily. Some experts argued that keeping thimerosal in the vaccine supply wasn’t worth the risk to children’s health—and to public trust in immunization. If the FDA publicized its findings on mercury and the government didn’t take action, “we would look pretty stupid or unconcerned,” Neal Halsey, who was at the time the chair of the American Academy of Pediatrics’ committee on infectious diseases, told me. Plus, thimerosal’s role in vaccines wasn’t technically essential: Its main use was to stave off harmful contamination in multidose vaccine vials, which clinicians repeatedly dip into; with some tinkering, many manufacturers would likely be able to sub in other preservatives, or switch to pricier single-dose containers.

So in 1999, the government and the AAP asked pharmaceutical companies to get rid of the thimerosal in their vaccines as quickly as they could—and advised health-care providers to delay giving the hepatitis B vaccine, which contained the compound, to low-risk newborns.

As it turned out, the compound never posed serious danger. The form of mercury in thimerosal is different from the one found in fish; scientists soon determined that it was excreted from the body faster—which meant that it didn’t pose equivalent risk. No major problems in childhood development could be linked to thimerosal-containing vaccines. At the time of the original decision, “if we’d had full knowledge, we wouldn’t have done it,” Orenstein told me. Thimerosal was, and is, safe.

But that wasn’t the message that anti-vaccine activists took away. Instead, they seized upon the government’s decision as an admission of guilt; multiple mercury-focused anti-vaccine activist groups sprang up. Some of them began to insist, without evidence, that thimerosal caused autism; among the most prominent advocates for that claim was Kennedy himself. The fervor around autism “caught us all by surprise,” Halsey told me. “That’s not what our concern was in 1999.”

And yet, those fears ballooned. In the mid-aughts, several states restricted thimerosal-containing vaccines for children and pregnant women. In some parts of the country, the misinformation yielded misguided treatments: In 2005, a family in Pennsylvania had their 5-year-old autistic son injected with a mercury-chelating chemical in hopes of curing his condition; less than an hour later, the boy died of a heart attack.

By 2001, thimerosal had been removed from most vaccines for Americans under 6. But the compound’s disappearance had costs. Multidose vials are an especially cheap, efficient way to package vaccines; blacklisting thimerosal made many shots more expensive, Paul Offit, a pediatrician at Children’s Hospital of Philadelphia, told me. The speed of the decision spurred confusion too. Shaken by the call to remove thimerosal, some hospitals stopped offering the hepatitis B vaccine to newborns entirely; shortly after, a Michigan baby on a delayed vaccination schedule, born to a mother infected with the virus, died.

Certain scientists, including Offit, still consider the removal of thimerosal a mistake, not least because it made vaccines appear more suspicious. In a press release at the time, the AAP noted that “the current levels of thimerosal will not hurt children, but reducing those levels will make safe vaccines even safer”—a statement that appeared to validate thimerosal’s dangers. In an attempt to preserve public trust, the government instead broke it, Offit said. “They were meeting the anti-vaccine activists halfway.”

Now ACIP seems poised to make a concession to those same anti-vaccine groups. “The fact that it’s come up again is reason for some people to say, ‘Well, there was an issue,’” Edwards told me.

In response to a request for comment, an HHS spokesperson said, “The new ACIP committee is committed to evidence-based medicine, gold-standard science, and common sense. Its recommendations will be grounded in data, not ideology or opinion.” The spokesperson did not address questions about thimerosal specifically or the evidence for once again bringing it under scrutiny.

But the experts I spoke with weren’t optimistic about the forthcoming discussion. In the past, any question the committee voted on was usually published weeks in advance, and subcommittees including ACIP members, CDC officials, and independent subject-matter experts vetted evidence and discussed policy options in advance of meetings, Grace Lee, a Stanford pediatrician who formerly chaired ACIP, told me. The new ACIP panel has had no time for that level of preparation. At least one new member, Vicky Pebsworth, has also argued that thimerosal-containing vaccines are dangerous for children and pregnant people in an article published by Children’s Health Defense. And on Tuesday, the night before the meeting began, Kennedy shared a lengthy post on X about thimerosal, citing outdated research, denying the existence of sound studies confirming the safety of thimerosal-containing vaccines, and criticizing “pharma-financed mainstream media’s mantric ritual of dutifully parroting the propaganda tropes spoon-fed them by vaccine makers and their captive regulators.”

The exact proposal that ACIP will vote on hasn’t yet been made public, either. But materials now posted to the CDC’s website hint at the question the group might consider. Redwood’s presentation, which was officially added to the agenda only on Tuesday, includes a series of slides that largely ignores the strong evidence supporting thimerosal-containing vaccines’ safety, misrepresents at least one study, and concludes that “removing a known neurotoxin from being injected into our most vulnerable populations is a good place to start with Making America Healthy Again.” In an unusual move, though, the materials pertaining to Redwood’s presentation also include a CDC report—flagged as “CDC background briefing material,” flanked with asterisks—that reiterates thimerosal’s safety, and the evidence that debunks a link to autism. (Redwood, Pebsworth, and the CDC did not respond to a request for comment.)

Even Senator Bill Cassidy—the chair of the Senate’s health committee, who helped secure Kennedy’s confirmation—seems to be having doubts about these developments. On Monday, he wrote on social media that the new ACIP lacked the expertise to make sound decisions about vaccines, and called for the meeting to be delayed “until the panel is fully staffed with more robust and balanced representation.” (A spokesperson for Cassidy did not respond to a request for comment.)

If ACIP does vote to remove recommendations for remaining thimerosal-containing vaccines, it could create practical problems, Halsey told me. Even though only a minority of flu vaccines would be affected, forcing manufacturers to alter their products on a tight timeline could make it harder to prepare for annual vaccination campaigns. Lower-resourced regions might also struggle to afford single-dose vials.

But the bigger issue with that decision would be this new committee’s brazen disregard for decades of evidence on thimerosal’s safety. The original discussion to remove thimerosal was contentious but understandable: a precaution taken in a vacuum of information. This time around, though, the experts have long had the knowledge they need—enough of it that there should be no discussion or vote at all.

https://www.theatlantic.com/health/archive/2025/06/vaccines-advisory-committee-acip-thimerosal-autism/683317/?utm_source=feed