Yes, some vaccines contain aluminum. That’s a good thing.

Robert F. Kennedy Jr. has often focused his criticism of vaccines on a common ingredient: aluminum, which he says is responsible for a litany of childhood illnesses, including food allergies, autism and depression.

“You wonder why a whole generation of kids are allergic to things,” Mr. Kennedy said in a 2021 interview. “It’s because we’re causing allergies, by filling them with aluminum.”

For many vaccine scientists, aluminum is a strange target. It is one of the most studied ingredients in vaccines, and perhaps in modern medicine.

“A tremendous amount of information is being collected,” said Dr. Andrew Racine, a pediatrician and chief medical officer at Montefiore Health System. “If there was something that stood out about the lack of security, we probably would have seen it somewhere, but it’s just not showing up.”

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

Still, as Mr. Kennedy prepares for confirmation hearings as Secretary of Health and Human Services, many experts fear that this expectation will face new scrutiny and may even be used as justification to limit access to several childhood vaccines, such as vaccines against hepatitis B. and pneumococcal disease.