The Truth About Mercury in Vaccines

One of the biggest reasons why parents are hesitant to vaccinate their children is fear of mercury in the shots. And that’s an understandable argument: For years, vaccines did contain thimerosal, a mercury-based preservative used to thwart the growth of bacteria and fungi in vaccine vials.

However, thimerosal was removed from most childhood vaccines in 2001. Research has found there are no adverse health effects. To help alleviate some fears about immunizations, read on to learn the truth about mercury in vaccines.

Is There Mercury in Vaccines?

Except for some flu shots, childhood vaccines no longer contain thimerosal. However, if you want to avoid mercury, you have options regarding the flu shot: You can ask for a vaccine that doesn't contain thimerosal.

For the most part, vaccines combine the following types of ingredients:

• Antigens to help your body build immunity
• Stabilizers and other ingredients to help vaccines stay safe and effective for a long time
• Ingredients, like aluminum salt, to help your body respond to the vaccine

But they also contain (or previously contained) some ingredients that worry parents. One of those ingredients was thimerosal, a preservative containing ethylmercury (a type of mercury cleared from the body more readily than methylmercury, the kind of mercury found in fish).

Why Are People Concerned About Mercury in Vaccines?

Parents have had many fears about mercury in vaccines, from a concern that mercury exposure could lead to learning disabilities to immune system and organ damage, to nervous system problems.

Learning disabilities

Fears about mercury stemmed from research showing that exposure to high doses can cause learning disabilities and damage the brain. This is why mercury is often blamed by those who suggest a connection between vaccines and autism.

However, older studies published a few years after thimerosal was removed from vaccines found the opposite to be true. A study in Pediatrics found that exposure at 3 months was inversely associated with the following problems at older ages:

• Hyperactivity
• Conduct problems
• Motor development
• Difficulties with sounds
• Speech therapy
• Special needs

Over-exposure

There's also the fear that mercury can compromise the immune system and kidneys. Researchers worried that, since children received multiple thimerosal-containing vaccinations in a short timeframe, their bodies were being overexposed.

Scientists later discovered that the ethyl-based mercury used in vaccines doesn't pose the same risk as methyl-based mercury (a known toxin found in fish that can damage the central nervous system). "Studies suggest that this is because the body can excrete ethyl-mercury more easily," says Dan Salmon, MD, associate director for policy and behavioral research at the Institute for Vaccine Safety at Johns Hopkins School of Public Health in Baltimore.

Even so, eliminating thimerosal from most vaccines is wise since organic mercury compounds, including ethylmercury, are recognized as neurotoxic agents. Limiting their use is important for preventing toxicity.

Nervous system

Ethylmercury can still enter the central nervous system. So, to reduce exposure to all forms of mercury, the American Academy of Pediatrics (AAP) recommended in July 1999 that thimerosal be removed from vaccines—just to be safe.

Thimerosal was effectively eliminated from almost every shot by 2001 as a precaution, says Neal Halsey, MD, a pediatrician and director of the Institute for Vaccine Safety at Johns Hopkins University in Baltimore.

The multi-dose flu vaccine for children contains a tiny amount of thimerosal, but it's considered safe. Parents can also opt for the single-dose vaccine instead.

Does Mercury in Vaccines Cause Autism?

One of the main concerns about thimerosal was its supposed connection to autism. But ever since the mercury was removed from vaccines, experts and researchers have consistently found no link between the two.

Take a 2008 study published in the Archives of General Psychiatry, which explored cases of autism in California. If thimerosal really did cause autism, you'd expect cases to decline after it was removed from vaccines in 2001. However, the opposite result proved true: Autism rates continued to rise after mercury in vaccines was eliminated.

Scientists and researchers concluded, then, that other factors must cause autism, says Eric Fombonne, MD, research professor of pediatrics at Oregon Health and Science University and joint editor of the Journal of Child Psychology and Psychiatry.

Studies in countries like Canada and Denmark proved similar results. For example, Demark removed thimerosal from vaccines in 1992, yet the country's autism rates continued to climb. Similarly, Canada has seen increases in autism rates, even after the government removed thimerosal from childhood vaccines.

Nine studies from the Centers for Disease Control and Prevention (CDC) between 2003 and 2012 also found autism on an upward trend after removing thimerosal from vaccines.

Two other prominent organizations—the World Health Organization and the Institute of Medicine—also found no link between mercury in vaccines and autism.