Does mercury make vaccines hazardous?

Dr. Suzanne Humphries says that if you drop a vial of mercury on the floor, you would need to call a special group of people to clean up the hazardous spill. She says since vaccines contain mercury, they are just as hazardous.

• Thimerosal helps prevent germs from growing in some multi-dose vaccines. 
• Thimerosal contains ethylmercury, which leaves the body quickly, unlike the mercury found in some fish.
• Research supports its safety in vaccine amounts and has not found a link to autism or harm to brain development.

The safety data sheet from GlaxoSmithKline gives directions for cleaning up spilled flu vaccine. For small spills, it says to wipe up the liquid with a cloth. These directions are for workplace cleanup. They do not, by themselves, tell us whether an ingredient is safe to inject.

Thimerosal is a preservative. It helps stop bacteria and fungi from growing in vaccine bottles that hold more than one dose. Each time a needle enters the bottle, germs could get inside.

Most vaccines do not contain thimerosal. It is still used in some multi-dose vaccines, including some vaccines used in other countries. These bottles can help keep vaccination costs down and take up less space in refrigerators. Thimerosal prevents germs from growing; it does not replace the need to keep vaccines at the right temperature.

U.S. flu vaccine guidance changed in 2025. Federal recommendations now call for children and adults to receive single-dose flu vaccines without thimerosal. This change in guidance does not mean thimerosal was found to be unsafe.

Some people worry about thimerosal because it contains mercury. But different forms of mercury act differently in the body. Thimerosal breaks down into ethylmercury, which leaves the body quickly. This differs from methylmercury, found in some fish, which stays in the body longer.

Research in babies, including those born early or with low birth weight, found that the amount of ethylmercury in the blood drops by half in about three to seven days. It leaves the body mainly through stool. Blood mercury levels return to their starting level within about a month.

Research has not found a link between thimerosal in vaccines and autism or damage to brain development. Thimerosal was removed from most routine childhood vaccines in the United States by 2001 as a precaution, not because harm had been proven. Autism diagnoses continued to rise afterward.

The evidence supports the safety of thimerosal in the small amounts used in vaccines. Its name and the presence of mercury alone do not tell us how it affects the body.