Most vaccines do not contain mercury. The only vaccines that still use a mercury-based preservative called thimerosal are certain multi-dose vials of the flu shot, which contain about 25 micrograms of mercury per dose. Every routine childhood vaccine in the U.S. has been available without thimerosal since the early 2000s, and single-dose flu shots and nasal spray versions are thimerosal-free as well.
What Thimerosal Is and Why It’s There
Thimerosal is a preservative that’s roughly 50% mercury by weight. It’s added to multi-dose vaccine vials to prevent bacteria and fungi from contaminating the liquid after the first needle puncture. Single-dose vials and pre-filled syringes don’t need a preservative because they’re used once and discarded.
A small number of flu vaccines still use it. According to the FDA, three multi-dose flu vaccines currently contain thimerosal at a concentration of 0.01%, delivering between 12.25 and 25 micrograms of mercury depending on the dose size. One tetanus-diphtheria vaccine also has a trace amount left over from manufacturing, though it’s not used as a preservative in the final product.
Why Vaccine Mercury Differs From Fish Mercury
The mercury in thimerosal is ethylmercury, which behaves very differently in your body than the methylmercury found in seafood. Methylmercury accumulates in tissue over time and can take weeks to clear. Ethylmercury breaks down much faster. In animal studies, its half-life in blood was roughly 9 days, meaning most of it was excreted within a month. What remained was found primarily in the liver and kidneys, and its behavior more closely resembled inorganic mercury than the organic methylmercury that builds up in fish.
This distinction matters because early safety concerns about thimerosal were based on what scientists knew about methylmercury toxicity, since data on ethylmercury was limited at the time. Subsequent research showed the two compounds don’t pose the same risk. A single serving of canned albacore tuna contains roughly 40 to 50 micrograms of methylmercury, which lingers in the body far longer than the 25 micrograms of ethylmercury in a flu shot.
How Thimerosal Was Removed From Childhood Vaccines
In July 1999, the American Academy of Pediatrics and the U.S. Public Health Service jointly recommended removing thimerosal from children’s vaccines as a precautionary measure. The FDA then worked with manufacturers to reformulate vaccines, and by the early 2000s, all routine childhood vaccines were available in thimerosal-free versions. This decision was not driven by evidence of harm but by a general principle of reducing mercury exposure wherever feasible.
The latest regulatory shift came in 2025. Following a 5-to-1 vote by the CDC’s Advisory Committee on Immunization Practices, HHS Secretary Robert F. Kennedy Jr. signed a recommendation to remove thimerosal from all U.S. influenza vaccines. The recommendation specifies that children, pregnant women, and adults should receive only single-dose flu vaccines free of mercury.
What the Safety Evidence Shows
Multiple large-scale studies have examined whether thimerosal in vaccines causes health problems, including autism and developmental delays. None have found evidence of harm at the doses used in vaccines. The World Health Organization’s Global Advisory Committee on Vaccine Safety has reviewed the data repeatedly and concluded there is no evidence of mercury toxicity in infants, children, or adults from thimerosal-containing vaccines. Their current position is that there is no scientific basis for changing vaccination recommendations on safety grounds.
The American College of Obstetricians and Gynecologists has similarly stated that thimerosal-containing flu vaccines are safe during pregnancy, citing multiple studies that found no health or developmental problems in children born to women who received those vaccines. ACOG does not recommend one flu vaccine formulation over another for pregnant patients.
How to Get a Mercury-Free Vaccine
If you want to avoid thimerosal entirely, your options are straightforward. Every routine childhood vaccine is already thimerosal-free. For the flu shot, ask your pharmacy or doctor’s office for a single-dose vial or pre-filled syringe rather than a dose drawn from a multi-dose vial. The nasal spray flu vaccine also contains no preservative. With the 2025 ACIP recommendation now in place, multi-dose thimerosal-containing flu vaccines are expected to phase out of U.S. distribution, making mercury-free options the default.