Vaccine Education Center

Vaccines and Thimerosal

Do vaccines contain thimerosal?

Preservatives are used in some vaccines to prevent bacterial or fungal contamination. The requirement for preservatives in vaccines arose from many incidents in the early 20^th century of children who developed severe and occasionally fatal bacterial infections after administration of vaccines contained in multi-dose vials. For example, in 1916, four children died, 26 developed local abscesses, and 68 developed severe systemic infections after receipt of a typhoid vaccine contaminated with the bacteria Staphylococcus aureus. As a consequence of this and similar incidents, preservatives have been required for vaccines contained in multi-dose vials (with some exceptions) since the 1930s.

Thimerosal, a mercury-containing preservative, has been the focus of intense scrutiny by the U.S. Congress and the news media. Thimerosal is no longer used as a preservative in any childhood vaccine with the exception of the influenza vaccine. Attention by the news media has caused some parents to fear that thimerosal contained in vaccines might have harmed their children.

Removal of thimerosal from vaccines

Removal of thimerosal from vaccines was precipitated by an amendment to the Food and Drug Administration (FDA) Modernization Act that was signed into law on November 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 lessening human exposure to mercury, a known neurotoxin and nephrotoxin.

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 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.

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.

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 a thousand 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 one hundred thousand 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.
• The Centers for Disease Control and Prevention published a study by Bill Thompson and colleagues in the New England Journal of Medicine in 2007. The study compared results of forty separate tests performed on one thousand 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 fifteen times the quantity of mercury contained in the influenza vaccine.

What is known about the causes of autism?

• First, like cystic fibrosis or sickle-cell disease, autism clearly 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 clearly has a genetic basis, environmental factors can also cause the disease. For example, children whose mothers took thalidomide 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.

Selected References

A. Hviid, et al., “Association Between Thimerosal-Containing Vaccine and Autism,” Journal of the American Medical Association 2003; 290:1763-1766.

T. Verstraeten, et al., “Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Databases,” Pediatrics 2003; 112:1039-1048.

J. Heron, J. Golding, and ALSPAC Study Team. “Thimerosal Exposure in Infants and Developmental Disorders: A Prospective Cohort Study in the United Kingdom Does Not Show A Causal Association,” Pediatrics. 2004; 114:577-583.

N. Andrews, et al., “Thimerosal Exposure in Infants and Developmental Disorders: A Retrospective Cohort Study in the United Kingdom Does Not Show A Causal Association,” Pediatrics, 2004; 114:584-591.

Fombonne, E., et. al. "Pervasive Developmental Disorders in Montreal, Quebec, Canada: Prevalence and Links with Immunizations," Pediatrics. 2006; 118:139-150.

Chess S, Fernandez P, Korn S. “Behavioral Consequences of Congenital Rubella,” J Pediatr, 1978; 93:699-703.

Deykin EY, MacMahon B. “Viral Exposure and Autism,” Am J Epidemiol 1979; 109:628-638.

Rodier PM. “The Early Origins of Autism,” Scientific American February 2000, pp.56-63.

Stomland K, Nordin V, Miller M, et. al. “Autism in Thalidomide Embryopathy: A Population Study,” Developmental Med Child Neurol 1994; 36:351-356.

Nelson KB, Bauman ML. “Thimerosal and autism?” Pediatrics 2003:111:674-679.

Gundacker C, Pietschnig B, Wittmann KJ, et al. “Lead and Mercury in Breast Milk,” Pediatrics 2002; 110:873-878.

Pichichero ME, Cernichiari E, Lopreiato J, Treanor J. “Mercury Concentrations and Metabolism in Infants Receiving Vaccines Containing Thimerosal: A Descriptive Study,” Lancet 2002; 360:1737-1741.

Stehr-Green, P, “Autism and Thimerosal-Containing Vaccines: Lack of Consistent Evidence for an Association,” Am J Prev Med 2003; 25:101-106.

Madsen, K. “Thimerosal and Occurrence of Autism: Negative Ecological Evidence from Danish Population-Based Data,” Pediatrics 2003; 112:604-606.

Thomson, B, Price, C. et. al. “Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years,“New England J of Med 2007; 357:1281-1292.

Schechter, R. and Grether, J. “Continuing Increases in Autism Reported to California's Developmental Services System: Mercury in Retrograde, “Arch Gen Psychiatry, 2008; 65:19-24.

Reviewed by: Paul A. Offit, MD
Date: April 2013