The truth about thimerosal

Vaccinations have been around for over 200 years and vaccine rejectionists have been around for nearly that long. Over the years, the basis for claims of harm from vaccines have changed, but one factor has remained constant. Vaccine rejectionists have never been right. The current fear mongering surrounding thimerosal is just the latest iteration in an ongoing effort. That fear mongering rests on two factors, ignorance of basic chemistry and ignorance of the existing research on thimerosal.

To hear vaccine rejectionists tell it, all mercury containing compounds are dangerous and therefore thimerosal is dangerous. But that’s not how chemistry works. The toxicity of a substance depends on how atoms are arranged, not simply which atoms are present. The fact that some mercury compounds are dangerous does not mean that thimerosal must be dangerous because it contains mercury.

Consider the example of sodium. Sodium is both poisonous and explosive. That does not mean that compounds that contain sodium are either poisonous or explosive. Table salt (sodium chloride) contains sodium, yet we do not worry that salting our food will result in an explosion at the dinner table.

Similarly, mercury is poisonous, but that does not mean that any compound that contains mercury is also poisonous. Some mercury containing compounds, like methyl mercury, are poisonous. Methyl mercury is the found in fish and is the reason why restrictions of fish consumption are recommended for certain groups. Thimerosal is ethyl mercury and is not poisonous.

Though methyl mercury and ethyl mercury might sound like they are very similar, one is poisonous and the other is not. How can that be? Consider the case of alcohol. Methyl alcohol (methanol), also known as wood alcohol, will lead to blindness or death if you drink it. Ethyl alcohol (ethanol) is the alcohol found in wine and spirits. Chemical structure is more important than the identity of the individual atoms that make up the compound.

The safety of thimerosal is more than simply theoretical. Contrary to the claims of vaccine rejectionists, thimerosal has been studied extensively in large populations. There have been many studies that demonstrate the safety of thimerosal.

Consider the study Thimerosal exposure in infants and developmental disorders: a retrospective cohort study in the United Kingdom does not support a causal association (Andrews et al. Pediatrics 114. 584-591.2004). This was a retrospective cohort study was performed of 109 863 children who received thimerosal containing DPT vaccines. The study found no evidence that thimerosal caused developmental delays.

Thimerosal exposure in infants and developmental disorders: a prospective cohort study in the United kingdom does not support a causal association (Heron et al. Pediatrics 114. 577-583.2004) followed 14,000 children for up to 7+ years. The authors found:

Contrary to expectation, it was common for the unadjusted results to suggest a beneficial effect of thimerosal exposure. For example, exposure at 3 months was inversely associated with hyperactivity and conduct problems at 47 months; motor development at 6 months and at 30 months; difficulties with sounds at 81 months; and speech therapy, special needs, and “statementing” at 91 months… CONCLUSIONS: We could find no convincing evidence that early exposure to thimerosal had any deleterious effect on neurologic or psychological outcome.

The bottom line is that the vaccine rejectionists are wrong once again. The fact that thimerosal contains mercury does not mean it is dangerous because the chemical structure determines the danger, not the identity of the individual atoms. More importantly, thimerosal has been studied extensively in large populations over time and has been demonstrated repeatedly to be safe.