What’s in the Vaccine? Aluminum, Mercury and Other Scares

“Read the ingredients” is one of the anti-vaccine movement’s oldest lines, because chemical names sound frightening. In 2025 it moved from Facebook to the U.S. government: the Health Secretary demanded that a journal retract the largest study ever done on aluminum in vaccines, and the President said aluminum should come out. Here is what the ingredients are, how much there is, and what the evidence says.

1. Aluminum

Some vaccines contain tiny amounts of aluminum salts, such as aluminum hydroxide or aluminum phosphate. They are adjuvants: they help the immune system respond, so fewer doses are needed. They have been used since the 1920s and are in vaccines against diphtheria, tetanus and whooping cough, hepatitis A and B, HPV and some others.

• How much? Aluminum is the most common metal in the earth’s crust, and every baby takes it in through breast milk, formula, food and water. Dr. Paul Offit estimates that vaccines add about 8 mg over all of childhood, against roughly 400 mg from everyday sources over 18 years (KFF Health News / CBS, Nov 2025).
• Does it build up? The kidneys excrete it. FDA researchers concluded the quantities in vaccines pose an “extremely low risk to infants” (FactCheck.org).
• Known side effect: occasionally, small itchy lumps at the injection site. Not autism, not allergies, not autoimmune disease.

2. The Danish study Kennedy wanted retracted

On 15 July 2025, Danish researchers led by Anders Hviid published in Annals of Internal Medicine the largest study ever done on this question: the health records of about 1.2 million children over two decades. They found no link between the amount of aluminum a child received from vaccines and any of 50 chronic conditions, including autoimmune, allergic and neurodevelopmental disorders such as autism.

• 1 August 2025: Health Secretary Robert F. Kennedy Jr. called the study “flawed science” with “bought conclusions” in an article for TrialSite News, and demanded it be retracted. He later called it a “deceitful propaganda stunt.”
• 11 August 2025: the journal refused to retract it. Hviid said the criticisms were not substantive.
• What Kennedy cited: a single result from the supplementary tables, a 67% higher risk of Asperger’s syndrome in one subgroup. FactCheck.org found it was one of more than 540 comparisons, based on 51 children, with a margin of error running from 1% to 177% — the kind of result that turns up by chance when you run hundreds of tests. The study’s main analysis found nothing.

Children’s Health Defense and allied writers have since published “critiques” of the study. None has changed its main finding.

3. Thimerosal (mercury)

Thimerosal is a preservative that contains ethylmercury, which the body clears quickly — unlike the methylmercury in some fish, which builds up. As a precaution, it was taken out of routine childhood vaccines in Canada and the United States around 2001. Autism diagnoses kept rising after it was removed, which is one of the clearest signs that it was never the cause.

In June 2025 Kennedy’s reconstituted ACIP nevertheless voted to recommend only thimerosal-free flu vaccines, and he approved it on 23 July 2025. Thimerosal was by then used only in multi-dose flu vials, which are cheaper and easier to use in mass clinics and in poorer countries. The vote changed little in practice but gave an old myth a federal stamp (PolitiFact).

4. Formaldehyde

Formaldehyde is used to inactivate viruses and toxins during manufacture, and trace amounts can remain. But the human body makes formaldehyde all the time as part of normal metabolism, and the amount already circulating in a baby’s blood is far greater than the trace in any vaccine. “It’s embalming fluid” is a scare line, not a dose.

5. “Fetal cells”

A few vaccines, including those for rubella, chickenpox and hepatitis A, are grown in cell lines that began from two elective abortions in the 1960s. The cells have been grown in laboratories ever since; no new fetal tissue is used, and the finished vaccine does not contain fetal cells. The Vatican’s Pontifical Academy for Life has said Catholic parents may use these vaccines in good conscience to protect their children and others.

6. How to answer “read the ingredients”