Vaccines do contain small amounts of metals, but the term “heavy metals” overstates what’s actually in them. The two metals most commonly associated with vaccines are aluminum and mercury. Aluminum salts are currently used in several vaccines as an ingredient that strengthens the immune response. Mercury, in the form of a preservative called thimerosal, was removed from childhood vaccines in the United States in 2001 and today only appears in certain multi-dose flu vaccine vials.
Aluminum in Vaccines
Aluminum salts are the most common metal-containing ingredient in vaccines today. They serve as adjuvants, meaning they boost the body’s immune response to the vaccine so it produces stronger, longer-lasting protection. Without an adjuvant, some vaccines simply wouldn’t work well enough. The types of aluminum salts used include aluminum phosphate, aluminum hydroxide, and amorphous aluminum hydroxyphosphate sulfate.
The amount of aluminum in a single vaccine dose ranges from about 133 to 1,340 micrograms, depending on the vaccine. The FDA caps it at 850 micrograms (0.85 mg) per dose for vaccines licensed in the United States. To put that in perspective, aluminum is one of the most abundant elements on Earth and is present in drinking water, fruits, vegetables, grains, and processed foods. The amount you take in through your diet on a typical day far exceeds what any vaccine delivers.
Not all vaccines contain aluminum. The MMR vaccine, the chickenpox vaccine, the nasal spray flu vaccine, and mRNA COVID-19 vaccines are all aluminum-free. Vaccines that do use aluminum adjuvants include those for hepatitis A, hepatitis B, DTaP (diphtheria, tetanus, pertussis), and pneumococcal disease.
How Aluminum Adjuvants Work
When a vaccine containing aluminum salts is injected, the aluminum holds onto the vaccine’s active ingredient and slows its release at the injection site. This gives immune cells more time to arrive, recognize the foreign material, and mount a response. The aluminum also triggers nearby cells to release chemical signals that recruit additional immune cells to the area. This cascade of activity is what makes the vaccine more effective at training your immune system to recognize a specific virus or bacterium.
Aluminum adjuvants have been used in vaccines for over 70 years. The body processes and eliminates the small amount of aluminum from a vaccine within days to weeks, primarily through the kidneys. Infants are exposed to far more aluminum through breast milk and formula over the course of their first six months than through the entire recommended vaccine schedule during that same period.
Mercury and Thimerosal
Thimerosal is a preservative that contains a form of mercury called ethylmercury. It was used for decades in multi-dose vaccine vials to prevent bacterial and fungal contamination. In 1999, when the FDA reviewed mercury exposure across childhood vaccines, it calculated that infants could receive up to 187.5 micrograms of mercury from three different vaccine series by six months of age. Although no evidence showed this caused harm, the CDC and the American Academy of Pediatrics jointly recommended removing thimerosal from childhood vaccines as a precaution. By 2001, it was gone from all routine childhood vaccines in the U.S.
Today, thimerosal only remains in some multi-dose flu vaccine vials, where it prevents contamination once the vial has been punctured multiple times. A single dose from these vials contains about 25 micrograms of mercury, roughly the same amount found in a can of tuna. Single-dose flu shots and the nasal spray flu vaccine contain no thimerosal at all, so if you prefer a thimerosal-free option, you can request one.
Ethylmercury vs. Environmental Mercury
The mercury in thimerosal (ethylmercury) behaves very differently from the mercury that accumulates in fish and the environment (methylmercury). Methylmercury builds up in the body over time because it’s slow to clear. Ethylmercury has a much shorter half-life in the blood and is excreted from the body far more quickly. This is an important distinction, because the safety concerns people associate with mercury exposure are based on methylmercury from environmental contamination and seafood consumption, not the type found in vaccines.
The original safety guidelines the FDA used to evaluate thimerosal in 1999 were designed for methylmercury, not ethylmercury, which made the numbers look more alarming than they turned out to be. Subsequent research confirmed that ethylmercury clears the body too quickly to accumulate to harmful levels from vaccine doses.
Newer Vaccines Use Different Ingredients
Several modern vaccines have moved away from aluminum adjuvants entirely, using newer immune-boosting ingredients instead. The shingles vaccine Shingrix uses a combination of a modified bacterial compound and a plant-derived molecule from tree bark. One version of the flu vaccine for older adults uses an oil-in-water mixture called MF59, made from squalene, a substance found naturally in the human body. A hepatitis B vaccine called Heplisav-B uses a synthetic DNA fragment to stimulate immune activity. These alternatives reflect advances in vaccine design, not safety problems with aluminum.
The Bottom Line on Metals in Vaccines
Aluminum is present in many vaccines and serves a functional purpose. The amounts are small, regulated, and well below what you encounter through food and water. Mercury in the form of thimerosal is no longer in any routine childhood vaccine and is only found in certain multi-dose flu vials, with thimerosal-free alternatives readily available. The metals in vaccines are measured in micrograms, processed by the body efficiently, and have extensive safety records spanning decades.