With further changes to the U.S.' recommended vaccine schedule likely in the year ahead under Health Secretary Robert F. Kennedy Jr.'s leadership, his agency's recent scrutiny of one vaccine additive in particular — aluminum salts — may offer a clue about what's to come.
Earlier this month, members of the Centers for Disease Control and Prevention's vaccine advisory committee —whom Kennedy selectedafter firing the previous group — suggested digging into concerns about aluminum salts, though large studies have found them to be safe. Andrew Nixon, a spokesperson for the Department of Health and Human Services, told NBC News that the committee is "reviewing the body of science related to aluminum and other possible contaminants in childhood vaccine[s]."
Similarly, a statement thatappeared on the CDC websitelast month said that HHS is investigating whether aluminum in vaccines could be linked to autism.
Aluminum salts are not a "contaminant" in vaccines: The compound is added as an adjuvant, an ingredient that boosts the body's immune response to a vaccine, allowing for a smaller dose to be used. Nearly a century of evidence has found it to be safe for this purpose. Aluminum salts are naturally present in soil and water, and the amount children are exposed to from vaccines is minuscule compared with cumulative daily exposures from food. (Exposure from infant formula or breastmilkis higher than exposure from vaccines in the first six months of life, as well.)
"This is not the thing that you wrap your food in at the barbecue. … The purpose of them is to just help the immune system respond a little more robustly to that vaccine," said Dr. Michelle Fiscus, chief medical officer at the Association of Immunization Managers, a nonprofit that supports public vaccination programs.
"Aluminum adjuvants have made vaccines very, very effective and have helped us significantly reduce suffering and sickness and death," she added.
Skeptical or negative statements about aluminum have cropped up repeatedly during federal health announcements and meetings in recent months. President Donald Trump in September said aluminum was being "taken out of the vaccines" during a news conference in whichhe and Kennedy warned that Tylenol use in pregnancymay be linked to autism. (The bulk of scientific research has not identified such a link.)
"Who the hell wants that pumped into a body?" Trump said of aluminum.
Then, during the CDC's vaccine advisory panel meeting, Dr. Tracy Beth Høeg, acting director of the Food and Drug Administration's Center for Drug Evaluation and Research, argued for adopting the vaccine schedule that Denmark uses, which is sparser than in the U.S., in part because doing so would reduce aluminum exposure.
"I do not feel like we have the data to show that there is an established safe amount [of aluminum] that children can receive before the age of 2, before the age of 18," Høeg said.
Some public health experts worry that the investigation into aluminum is part of a broader push to restrict access to or approval of some childhood shots.
Last week, HHS postponed a planned announcement about children's health to the new year. The details are not yet known —CNN reportedthat the Trump administration was considering decreasing the number of recommended childhood vaccines to align more closely with Denmark's, citing an unnamed source "familiar with the plans." Several other outlets subsequently reported the same, though NBC News has not confirmed that plan.
Already, the CDC's vaccine advisory committee voted tostop recommending the hepatitis B vaccinefor all newborns, and as a result, theCDC now advises womenwho test negative for the virus to decide about the shot with their medical providers.
Changing vaccine recommendations based on concerns about aluminum salts would be a flimsy justification, several public health experts said. Even in Denmark, many recommended vaccines contain aluminum salts, including those for human papillomavirus (HPV), pneumococcal disease, tetanus and whooping cough.
Of the pediatric vaccines on the CDC's schedule that aremissing from Denmark's universal recommendations, just three — for hepatitis A, hepatitis B and meningococcal vaccines — contain aluminum salts. The others — for respiratory syncytial virus (RSV), rotavirus, flu and chickenpox — do not.
The shot that anti-vaccine activists most frequently and erroneously associate with autism — the combination measles, mumps and rubella vaccine — also does not contain aluminum salts.
Astudy of more than 1.2 million children in Denmark, published in July in the journal Annals of Internal Medicine, found no link between aluminum salts from vaccines and neurodevelopmental disorders, including autism.
But Kennedy — who has made unsubstantiated claims that aluminum in vaccines is linked to rising rates of allergies and autism in children — demanded the paper be retracted.
"A closer look reveals a study so deeply flawed it functions not as science but as a deceitful propaganda stunt by the pharmaceutical industry," hewrote in an editorial on the website TrialSite News.
Kennedy argued that the paper excluded some children who may have been at risk and did not include a control group. However, Annals of Internal Medicine stood by the study and said there wasno reason to retract it.
Kennedy, a longtime critic of vaccines, had been involved in lawsuits against the pharmaceutical company Merck over allegations related to its HPV vaccine (which contains aluminum salts), but since taking office, he has said any fees earned would go to one of his sons. His focus on aluminum dates back to his time as chairman and chief litigation counsel atthe anti-vaccine group Children's Health Defense. The group has claimed for years that thimerosal — a mercury-based preservative — is linked to autism, and Kennedy said ina podcast appearance in 2020that aluminum had replaced thimerosal in some vaccines, causing them to remain toxic.
In reality, the ingredients serve different purposes and aluminum salts have been used in vaccines for nearly a century. Thimerosal, meanwhile, was largely phased out of childhood vaccines in 2001, and under Kennedy's leadership, HHS in Julypulled the ingredient from the roughly 5% of flu vaccinesthat still had it.
Arecent analysisfrom the World Health Organization found no link between autism and vaccines containing thimerosal or aluminum.
"There's this constant movement of the goal post to try to implicate vaccines in the development of these diseases, and there's just not science to back those claims," Fiscus said.
In 2021, Kennedytold food blogger Mikhaila Petersonthat all aluminum-containing vaccines had "negative risk profiles" and that the brains of children with autism were "loaded with aluminum." He suggested, too, that kids develop food allergies because "we've been inducing allergies by pumping them full of aluminum."
Alarge German studyin 2011, however, did not find an increased risk of allergies in vaccinated children and even identified a decrease in hay fever among the group. In 2023,a study founda positive association between vaccine-related aluminum exposure and persistent asthma, but the results couldn't be replicated and scientists said the research didn't properly control for confounding variables.
In response to mounting hesitancy about aluminum salts in vaccines, Dr. Seth Ari Sim-Son Hoffman, a physician-scientist at Stanford Medicine, decided to reanalyze the existing data with some of his colleagues. The team's findings,published this monthin the journal Pediatrics, found no major safety concerns with aluminum-containing vaccines. Side effects were mostly limited to redness and swelling at the injection site.
"When you see the same 'no' finding or 'no' association across multiple countries, multiple study designs and over a million children, that's really, really clear and reassuring," Hoffman said.
The current childhood vaccine schedule in the U.S., he added, "is backed by powerful evidence in terms of safety and effectiveness."
