Yes, Some Vaccines Contain Aluminum. That’s a Good Thing.

R.F.K. Jr. and others have blamed the ingredient for allergies and other illnesses. Scientists say it actually bolsters the immune response.

Robert F. Kennedy Jr. has often trained his criticisms of vaccines on a common ingredient: aluminum, which he has suggested is responsible for a litany of childhood ailments, including food allergies, autism and depression.

“You wonder why a whole generation of children is allergic to stuff,” Mr. Kennedy said during a 2021 interview. “It’s because we’re inducing allergies, pumping them full of aluminum.”

To many vaccine scientists, aluminum is a strange target. It is among the most studied ingredient in vaccines, and perhaps in modern medicine.

“There’s a huge amount of information that’s gathered,” said Dr. Andrew Racine, a pediatrician and chief medical officer at the Montefiore Health System. “If there was something jumping out about a lack of safety, we would most likely have seen it someplace, and it just doesn’t appear.”

Aluminum salts, a more soluble form of the metal, are added to vaccines like the Tdap shot to bolster the body’s immune response. The ingredient has repeatedly been evaluated as a part of vaccines in clinical trials and administered in billions of doses over several decades.

Even so, as Mr. Kennedy prepares for confirmation hearings as secretary of health and human services, many experts fear the standby will face fresh scrutiny, and may even serve as a justification for limiting access to several childhood vaccines, like the shots for hepatitis B and pneumococcal disease.