Why is aluminum in vaccines?

Since the 1930s, aluminum salts have been added to some pediatric vaccines to stimulate immune response, behaving as a beacon for the immune system to pinpoint invaders and prompt the body to produce antibodies against the virus or bacteria the vaccine aims to protect against. This is known as an adjuvant.

"You can't just have an antigen and have an immune response, you need some kind of stimulus to trigger that response," said Ross Kedl, a professor of immunology and microbiology at the University of Colorado Anschutz Medical Campus. "An adjuvant is a substance that alerts the body's immune response to the vaccine's antigen. Without adjuvants, you actually create tolerance, which is the opposite effect of what you want a vaccine to do."

In the United States, aluminum salts are used in vaccines for diphtheria, tetanus and pertussis, pneumonia, HPV, and hepatitis B. These aluminum salts are injected into the muscle and the majority are cleared out by the body and filtered through the kidneys within two weeks; however, small amounts can linger for years.

"The aluminum that is in vaccines is in the form of extremely small amounts of aluminum salts which is not the same as elemental aluminum which is a metal," said Anders Hviid, lead author of the study and head of the department of epidemiology research at the Statens Serum Institut in Denmark. "It's really important for parents to understand that we are not injecting metal into children."

Aluminum salts have attracted the attention of vaccine opponents who have argued they may overstimulate immune systems. HHS Secretary Robert F. Kennedy Jr. said on a podcast in 2024 that aluminum in vaccines is "extremely neurotoxic."

The researchers found no evidence that exposure to aluminum salts in vaccines led to any statistically significant increase in a child's risk of developing any of the 50 conditions examined, which included asthma and autism. None of the conditions occurred at statistically higher levels than would normally be expected, which ruled out any moderate or substantially increased risk from exposure to aluminum salts in vaccines.

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Hviid said the findings were "quite striking."

"We can exclude meaningful increases with a large degree of certainty for many of these outcomes," he said. "We should not be concerned about aluminum used as an adjuvant in childhood vaccines. I think that's the core message."

Kedl also praised the study, noting that Scandinavian public health studies are of uniquely good quality.

"[This excellence is] partly because they have, for a long time, had such a unified health system," Kedl said. "Everyone is tracked for life from birth and you can go back for many years and ask, 'Can we find a link between something that happened in the past and in the future?'"

Paul Offit from Children's Hospital of Philadelphia said the study "answers a lot of questions," also saying that the strength of the study lies in Denmark's national healthcare system. "The access to data is much easier in that system than it is here, where data are more fragmented," he said.

Anna Durbin, director of the Center for Immunization Research at Johns Hopkins Bloomberg School of Public Health, said she was "very encouraged by the data, particularly for asthma as I know vaccine hesitancy groups are concerned by this."

Edward Belongia, who conducted studies on vaccine effectiveness and safety for decades and was a senior research scientist at the Marshfield Clinic Research Institute before his recent retirement, said the new study is "the largest and most definitive observational study on the safety of vaccine-related aluminum exposure in children" ever conducted.

"This is a rigorous and well-designed study that should put to rest any lingering doubts about the potential risks to children from cumulative aluminum exposure in vaccines," Belongia added.

Matthew Daley and Jason Glanz, two researchers who authored a previous U.S.-based study suggesting aluminum salts in childhood vaccines could be tied to an increased risk of developing asthma before the age of 5, welcomed the new study and said it was well done.

Specifically on the topic of asthma risk, Daley said the new study should give parents confidence that vaccinating their children according to the recommended schedule shouldn't increase their risk of developing the condition.

"As a practicing pediatrician, I find that reassuring, and my patients and other clinicians I work with should find that reassuring," said Daley, who noted that even after the study he coauthored was published, he continued to urge parents to vaccinate their children as he felt the benefits of vaccination were greater than the small potential risk his study identified.