Some people have worried that thimerosal, an ethylmercury-containing preservative in some multi-dose preparations of influenza vaccine, could cause mercury poisoning either in children or affect the unborn children of pregnant women who receive this vaccine. But, for many reasons, thimerosal contained in vaccines is not harmful.

All mercury isn’t the same: Methylmercury vs. ethylmercury

Mercury is a naturally occurring element found in the earth's crust, air, soil and water. Since the earth's formation, volcanic eruptions, weathering of rocks and burning of coal have caused mercury to be released into the environment. Once released, certain types of bacteria in the environment can change mercury to methylmercury. Methylmercury makes its way through the food chain in fish, animals and humans. At high levels, it can be toxic to people.

Thimerosal contains a different form of mercury called ethylmercury. Studies comparing ethylmercury and methylmercury suggest that they are processed differently in the human body. Ethylmercury is broken down and excreted much more rapidly than methylmercury. Therefore, ethylmercury (the type of mercury in the influenza vaccine) is much less likely than methylmercury (the type of mercury in the environment) to accumulate in the body and cause harm.

Because the names of these two chemicals vary by only a single letter, it is difficult to believe they could be very different; however, if you think about ethanol and methanol (also known as ethyl alcohol and methyl alcohol), you will see that the difference can be dramatic – ethanol is what we drink at a party, whereas methanol, added to some types of gasoline and antifreeze, is poisonous if consumed.

Learn more about thimerosal by watching this short video, part of the Talking about Vaccines with Dr. Paul Offit video series.

Removal of thimerosal from vaccines

Thimerosal was removed from vaccines after an amendment to the Food and Drug Administration (FDA) Modernization Act was signed into law on Nov. 21, 1997. The amendment gave the FDA two years to "compile a list of drugs and foods that contain intentionally introduced mercury compounds and ... [to] provide a quantitative and qualitative analysis of the mercury compounds in the list..." The amendment arose from a long-standing interest in reducing human exposure to mercury, a known neurotoxin (a substance that harms the nervous system) and nephrotoxin (a substance that harms the kidneys).

At the time the FDA Modernization Act was passed, infants were recommended to receive three different vaccines that contained thimerosal — diphtheria-tetanus-acellular pertussis (DTaP), hepatitis B and Haemophilus influenzae type b (Hib). Infants receiving all of these vaccines could have been exposed to a cumulative dose of mercury as high as 187.5 micrograms (ug)* by 6 months of age. The cumulative dose exceeded guidelines recommended by the Environmental Protection Agency (EPA) (see table below). Thimerosal, as a preservative, is no longer contained in any childhood vaccine, with the exception of the influenza vaccine.

*Note: One microgram is one-millionth of a gram. One gram is the weight of one-fifth of a teaspoon of water.

Micrograms = ug; milligrams = mg

EPA

• 5th percentile body weight: 65 ug
• 50th percentile body weight: 89 ug
• 95th percentile body weight: 106 mg

ATSDR

• 5th percentile body weight: 194 ug
• 50th percentile body weight: 266 ug
• 95th percentile body weight: 319 mg

FDA

• 5th percentile body weight: 259 ug
• 50th percentile body weight: 354 ug
• 95th percentile body weight: 425 mg

Exposure limits for mercury in infants less than or equal to 6 months of age by percentile body weight established by the Environmental Protection Agency (EPA), the Agency for Toxic Substance Disease Registry (ATSDR), and the Food and Drug Administration (FDA)

Thimerosal and autism

Evidence that mercury doesn't cause autism

Despite concerns, several studies have now proven that thimerosal in vaccines did not cause autism:

• A study published in the American Journal of Preventive Medicine in 2003 by Paul Stehr-Green found that more children in Sweden and Denmark were diagnosed with autism after removal of thimerosal from vaccines in those countries.
• Kreesten Madsen reviewed the medical record of 1,000 autistic children diagnosed between 1971 and 2000 and found that despite removal of thimerosal from vaccines, the incidence of autism diagnoses increased between 1992 and 2000. The study was published in Pediatrics in 2003.
• Similarly, Anders Hviid studied Danish children between 1990 and 1996 and found that the number of children with autism increased after thimerosal was removed from vaccines. This study was published in 2003 in the Journal of the American Medical Association.
• A study of more than 14,000 children in the United Kingdom found no evidence of increased autism relative to exposure to thimerosal. The study, by Jon Heron, was published in 2004 in the journal Pediatrics.
• Another study in the same issue of Pediatrics by Nick Andrews studied the records of more than 100,000 children who received different amounts of thimerosal and did not find evidence that exposure to thimerosal led to neurodevelopmental disorders.
• In 2004 the Institute of Medicine (IOM) reviewed more than 200 studies related to thimerosal and autism. They concluded that there was no relationship to cause concern.
• A study published in 2006 by Eric Fombonne looked at 28,000 children born in Canada between 1987 and 1998, a period during which there were varied amounts of thimerosal contained in recommended vaccines. Fombonne's study, published in Pediatrics, also concluded no relationship between quantity of thimerosal a child was exposed to and development of autism.
• Researchers at the Centers for Disease Control and Prevention published a study in The New England Journal of Medicine in 2007. The study compared results of 40 separate tests performed on 1,000 children exposed to thimerosal-containing vaccines and again, found no link between quantities of thimerosal and development of autism.
• Schechter and Grether published a study of rates of autism diagnosed in California between 1995 and 2007 and found that despite removal of thimerosal from vaccines, rates of autism have continued to increase. The study was published in Archives of General Psychiatry.
• In addition, several other pieces of information add to the reassurance provided by these studies:
  • In 1971 Iraq imported grain that had been fumigated with methylmercury. Farmers ate bread made from this grain. The result was one of the worst, single-source, mercury poisonings in history. Methylmercury in the grain caused the hospitalization of 6,500 Iraqis and killed 450. Pregnant women also ate the bread and delivered babies with epilepsy and mental retardation. But they didn't deliver babies with an increased risk of autism.
  • Studies of the head size, speech patterns, vision, coordination and sensation of children poisoned by mercury show that the symptoms of mercury poisoning are clearly different from the symptoms of autism.
  • Methylmercury is found in low levels in water, infant formula and breast milk. Although it is clear that large quantities of mercury can damage the nervous system, there is no evidence that the small quantities contained in water, infant formula and breast milk do. An infant who is exclusively breastfed will ingest more than twice the quantity of mercury that was ever contained in vaccines and 15 times the quantity of mercury contained in the influenza vaccine.

What’s known about the causes of autism

First, like cystic fibrosis or sickle-cell disease, autism has a genetic basis. Researchers found that when one identical twin had autism, the chance that the other twin had autism was about 90 percent; for fraternal twins, the chance was less than 10 percent.

Second, although autism has a genetic basis, environmental factors can also cause the disease. For example, children whose mothers took thalidomide, a medication that used to be prescribed for nausea during pregnancy, had birth defects, including malformed ears and shortened limbs. But they also had a significantly greater incidence of autism than babies born to mothers who never took thalidomide. Thalidomide clearly caused autism, but only if mothers took it early in pregnancy. If mothers took thalidomide in the second or third trimester of pregnancy, their babies weren't at increased risk of autism.

The thalidomide experience showed that there was a vulnerable time early in pregnancy when a drug could possibly cause autism. Echoes of the thalidomide story are found in babies infected with rubella virus. Babies born to mothers who suffered rubella early in their pregnancies develop birth defects involving the eyes, ears, brain and heart. They also are at greater risk of developing autism, but like thalidomide, only if the baby is exposed to rubella early during pregnancy. Babies don't develop autism if they are infected with the virus soon after birth. These findings suggest that a virus or a drug can cause autism, and that there is a vulnerable time early during pregnancy when the baby is at risk. However, during the second or third trimester of pregnancy, or after the child is born, the window for environmental factors causing autism has apparently closed.

Women in the United States also occasionally received mercury when they were pregnant. It happened when doctors found that the mother's blood type was not compatible with their baby's blood type. To prevent this blood mismatch from hurting the baby, mothers were given RhoGAM^®, a product that used to contain thimerosal as a preservative. However, consistent with the observation in Iraq, babies exposed to thimerosal in RhoGAM did not have a greater risk for autism than babies whose mothers never received RhoGAM. Although thalidomide and rubella virus can cause autism in pregnancy, scientific evidence clearly indicates that mercury doesn't.

Read more about autism and vaccines.