A new toxicological study adds to the growing evidence that the mercury-based vaccine preservative, thimerosal, is safe for use in children. I have blogged frequently about the myth that vaccines in general or thimerosal in vaccines is linked to autism. Last week I discussed that new evidence is allowing for the diagnosis of autism at a younger and younger age – potentially all the way back to birth – thereby falsifying the contention that vaccines are a cause of autism. I have also recently discussed that the removal of thimerosal from childhood vaccines in the United States did not result in a decrease in new autism diagnoses, as would be predicted if thimerosal caused or even contributed to autism rates. Meanwhile research is teaching us more and more about autism as a genetic disorder. Therefore epidemiological, historical, clinical, and genetic evidence is all pointing to the firm conclusion that thimerosal and vaccines generally are not a cause of autism or other neurological disorders.

While at present there is no significant scientific debate on this conclusion, there is an ongoing public debate driven mostly by ideological groups who have an anti-vaccine agenda. These groups rely upon increasingly implausible and desperate rationalizations, conspiracy mongering, and simply bad logic. But they have also based many of their arguments on a separate line of scientific evidence, that of toxicology – studying the effects of mercury and thimerosal on cells and in the body. In fact it is increasingly looking as if toxicological evidence will be the last stand for the “mercury militia” on this issue.

The reason for this is simple – mercury is a neurotoxin. No one denies that. Proponents of the thimerosal-autism hypothesis essentially argue that we can know that thimerosal causes autism because it is toxic to the brain, or at least we should assume that unless and until we prove beyond doubt that it is not toxic. This of course is an impossible standard and also we know the result of this question, mercury is a toxin.

But this arguments misses two key points. The first is that toxicological evidence can never answer the question of whether or not a substance actually did cause harm. In this respect epidemiological evidence trumps toxicological evidence – and all the epidemiological evidence says that thimerosal is not linked to any harm.

The second point is that toxicity is all about dose – any substance is safe in a small enough dose and anything is toxic (even oxygen, even water) in a high enough dose. What the anti-vaccine mercury militia has not demonstrated is that the mercury found in thimerosal is toxic at the doses given in vaccines. Related to the question of dose is that of access – is the toxin in question getting exposed to brain cells in a sufficient dose to cause neurotoxicity? Sure, mercury damages brain cells when you pour it on top of neurons in a petri dish, but this does not necessarily mean that thimerosal in vaccines causes a sufficient exposure of mercury to brain cells in people to cause toxicity. Because the epidemiological evidence shows no connection to autism, the assumption has been that the dose of mercury in vaccines is too small and it is cleared too quickly from the body for it to cause any measurable toxicity.

This brings us to the current study, which looked at blood, stool, and urine levels of mercury in children following their newborn, 2 month old, and 6 month old vaccinations. The study was performed in Argentina because, as I have said, thimerosal has been removed from all routine childhood vaccines in the US. The study, conducted by lead author Michael Pichichero from the University of Rochester, is titled: Mercury Levels in Newborns and Infants After Receipt of Thimerosal-Containing Vaccines and is published in this month’s issue of Pediatrics. They calculated the half-life of mercury in the blood of 72 healthy children in each age group and found that the mean half-life (the time it takes for half of remaining mercury to be cleared from the blood) is 3.7 days, without a significant difference between the age groups. Levels of mercury in the blood returned to pre-vaccination levels in about 30 days. These results are significant for various reasons.

First we need to know that there are various types of organic mercury, the two most important being methyl mercury (the type found in certain fish) and ethyl mercury (the type found in thimerosal). Methyl mercury is more toxic than ethyl mercury and is cleared more slowly, but data on the exact clearance rate for ethyl mercury was lacking. For this reason when calculating safety limits for human exposure the higher toxicity and slower clearance rates for methyl mercury were assumed. The half life of methyl mercury is about 30 days. This means that ethyl mercury (at least in young children, it is probably slower for adults) is cleared about 10 times more quickly than was previously assumed.

This is critical because the argument made for the potential toxicity of thimerosal was based upon the cumulative dose of ethyl mercury of all childhood vaccines. The argument was that ethyl mercury was building up from vaccine to vaccine, and that even if the dose from a single vaccine were safe the cumulative dose is what was toxic. This new study shows fairly conclusively that all the ethyl mercury given in one vaccine will be cleared by the time the next vaccine is due to be given – so their is no build-up, no cumulative dose. This effectively shoots down the last remaining scientific argument of the mercury militia.

The authors also noted that the levels of mercury they measured in the blood were generally low, writing:

We also observed that the highest levels in samples that were taken from children shortly after vaccination were <=8 ng/mL. The importance of blood levels of ethyl mercury for assessing toxicity is unknown, but blood levels have been shown to be a predictor of toxicity for methyl mercury exposure. The low levels of mercury detected in this study suggests relatively low risk for toxicity from this exposure.

But there are still unanswered questions, some legitimate and some desperate. The authors of the current study acknowledge the limitations of their evidence. They measured mercury levels in the blood, but did not separate methyl mercury from ethyl mercury. However, if anything this would tend to overestimate the amount of ethyl mercury remaining in the blood. They also measured mercury in the stool and urine. After vaccination mercury levels in the stool also rose and then slowly cleared, suggesting that the ethyl mercury from vaccines is primarily cleared by the liver and deposited into the feces. Levels did not significantly rise in the urine, which suggests that the kidneys are not a significant mode of clearance of ethyl mercury.

However, another possible interpretation of this evidence is that mercury was cleared from the blood but was not all directly cleared by the liver. Instead it rapidly spread to another compartment in the body (such as fatty tissue), and then was cleared from this second compartment by the liver. This is less likely given the rapidity with which mercury appeared in the stool, but it is not ruled out by this current study. A follow up study is required, one that will not only measure spot levels in the urine and stool but will examine 24 hour samples so that the actual amount of ethyl mercury that is being removed from the body can be estimated. This will better track where all the mercury is going and will help rule out the possibility that it is hiding for a time in another compartment of the body (and therefore not measured in the blood).

This is unlikely to change the ultimate conclusions of this study, but science works by considering all possibilities and then conducting experiments to eliminate all alternatives until the one true answer is all that remains.

The response from the mercury militia is predictable. They will claim, as they have in the past, that the average clearance of ethyl mercury in children may be rapid, but that certain individual children genetically have impaired mercury clearance. For these individuals the ethyl mercury from thimerosal will build up in the body from vaccine to vaccine, causing toxicity. At present there is no compelling evidence for this. I will also note that in the present study there were no individual children who had significantly different clearance than the average – no outliers with impaired mercury clearance – but the numbers of children in the study were too small to rule this out as an uncommon occurrence.

In conclusion, the toxicological evidence is moving in line with the epidemiological and other independent lines of evidence toward the conclusion that thimerosal given in childhood vaccines is not a contributor or cause of autism or other neurological disorder. There are more studies to be done, but there always are – science is a messy and complex business. But it is reassuring to know that children clear ethyl mercury 10 times more quickly than was previously assumed, and that levels do not build up from vaccine to vaccine. At least it is reassuring to those of us who look at all the evidence to formulate our opinions.