Thimerosal is a preservative that prevents bacterial and fungal contamination in multi-dose vaccine vials. It is roughly 50% mercury by weight, which is the reason it draws attention, but the form of mercury it contains behaves very differently in the body than the type found in contaminated fish. Today, thimerosal is found in only a handful of multi-dose flu vaccines in the United States. It was removed from virtually all routine childhood vaccines more than two decades ago.
Why Vaccines Need a Preservative
Single-dose vials and pre-filled syringes are sealed, used once, and discarded. Multi-dose vials are different. A healthcare worker punctures the rubber stopper with a new needle each time a dose is drawn out, and that repeated entry creates an opportunity for bacteria or fungi to get inside. Without a preservative, a contaminated vial could cause serious infections in every patient who receives a dose from it. Thimerosal has been used for this purpose since the 1930s because it is highly effective at killing microbes at very low concentrations, just 0.01% of the solution (1 part per 10,000).
The Mercury in Thimerosal
Thimerosal breaks down in the body into ethylmercury. This is a different compound from methylmercury, the form of mercury that accumulates in fish and is the basis for most mercury safety guidelines. The distinction matters because the two behave very differently once they enter your bloodstream.
Methylmercury is absorbed through the gut when you eat contaminated seafood and can linger in the body for weeks, gradually building up in tissues over time. Ethylmercury from a vaccine dose, by contrast, clears out far more quickly. A study of newborns and infants published in Pediatrics found that blood mercury levels returned to prevaccination levels within 30 days, with a calculated half-life of just 3.7 days. That rapid clearance means ethylmercury does not accumulate the way methylmercury does.
How Much Mercury Is in a Dose
A standard 0.5 mL dose of a thimerosal-containing flu vaccine delivers about 25 micrograms of mercury. For context, a typical 3-ounce can of light tuna contains roughly 30 to 40 micrograms of methylmercury, and that form sticks around in your body much longer. The smaller 0.25 mL pediatric dose of the same vaccine contains about half that amount, around 12.5 micrograms.
The FDA-approved flu vaccines that still use thimerosal as a preservative are all multi-dose presentations: Afluria, Flucelvax, and Fluzone. Each contains thimerosal at the 0.01% concentration. Single-dose pre-filled syringes and the nasal spray flu vaccine (FluMist) are thimerosal-free.
Why It Was Removed From Childhood Vaccines
In July 1999, the American Academy of Pediatrics and the U.S. Public Health Service issued a joint statement calling for the removal of thimerosal from vaccines routinely given to infants. The statement was explicit that no evidence existed of any harm caused by the low levels of thimerosal in vaccines. The move was described as a precautionary measure, driven by public concern about mercury exposure of any kind. Health officials reasoned that eliminating mercury from vaccines was a practical step they could actually take, unlike reducing mercury from harder-to-control environmental sources like food and water.
Manufacturers responded over the following years by reformulating childhood vaccines to use single-dose vials or alternative preservatives. By the early 2000s, thimerosal had been removed from or reduced to trace amounts in all routinely recommended childhood vaccines except certain multi-dose flu shots.
Thimerosal and Autism
The concern that thimerosal in vaccines could cause autism gained significant public attention in the early 2000s. In 2004, the Institute of Medicine (now the National Academies of Sciences, Engineering, and Medicine) completed a comprehensive review of the available evidence. The committee concluded that the body of epidemiological evidence “favors rejection of a causal relationship between thimerosal-containing vaccines and autism.” It also noted that the biological mechanisms proposed for how vaccines might cause autism were only theoretical, with no supporting evidence.
Since that review, thimerosal has been largely absent from childhood vaccines in the U.S. for over two decades, and autism diagnosis rates have continued to rise, further weakening any proposed link between the two.
Where Thimerosal Is Still Used
In the United States, thimerosal today appears only in certain multi-dose flu vaccine vials. If you want to avoid it entirely, you can request a single-dose pre-filled syringe or the nasal spray version, both of which are thimerosal-free. Most pharmacies and clinics stock these options.
Globally, the picture is different. Multi-dose vials are essential in large-scale vaccination campaigns in low- and middle-income countries because they are cheaper to produce, easier to transport, and require less refrigerator space. In those settings, thimerosal remains critical for keeping vaccines safe from contamination. The World Health Organization has concluded that the small amount of ethylmercury in multi-dose vaccines does not cause harm, and continues to support their use in global immunization programs.