Objective. There is an established link between exposure to mercury and impaired childhood cognitive development and early motor skills. Thimerosal (also known as thiomersal), a preservative used in a number of children's vaccines, contains ethylmercury (an organic compound of mercury), and there has been concern that this exposure to mercury may be of some detriment to young children. The aim of this research was to test in a large United Kingdom population–based cohort whether there is any evidence to justify such concerns.
Methods. We used population data from a longitudinal study on childhood health and development. The study has been monitoring >14 000 children who are from the geographic area formerly known as Avon, United Kingdom, and were delivered in 1991–1992. The age at which doses of thimerosal-containing vaccines were administered was recorded, and measures of mercury exposure by 3, 4, and 6 months of age were calculated and compared with a number of measures of childhood cognitive and behavioral development covering the period from 6 to 91 months of age.
Results. 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. After adjustment for birth weight, gestation, gender, maternal education, parity, housing tenure, maternal smoking, breastfeeding, and ethnic origins, we found 1 result of 69 to be in the direction hypothesized—poor prosocial behavior at 47 months was associated with exposure by 3 months of age (odds ratio: 1.12; 95% confidence interval: 1.01-1.23) compared with 8 results that still supported a beneficial effect.
Conclusions. We could find no convincing evidence that early exposure to thimerosal had any deleterious effect on neurologic or psychological outcome.
Comments
Re: Heron and Golding: erroneous premise, anomalous results - an update
Within a week of the last posting a new study appeared under NIH auspices which challenged the comparison of thimerosal toxicity by weight with that of methyl (environmental) mercury [1].
In relation to this I would like to make two points:
(i) The criterion for assessing the level of toxicity which I used in my commentary was that both mentioned and disregarded by Heron and Golding in their study.
(ii) Despite certain obscurities in the paper it is apparent that the levels of inorganic mercury that Burbacher et al discovered in the brains of macaque monkeys were actually worse than for an equal weight of methyl mercury.
I note as well that the Heron study was funded by the UK Department of Health. In a letter to the Sunday Times (July 15, 2001) I recently cited in another communication to Pediatrics [2] Dr Elizabeth Miller of the UK Public Health Laboratory referred to "theoretical concerns that the small amount of mercury in thiomersal could be harmful". To say that there is "a small amount of mercury in thiomersal" is even more unaccountable than Heron and Golding's "low doses" of mercury in DPT vaccine, and can only lead to the suspicion that the wool is being pulled over our eyes.
I find it troubling that Jon Heron and Jean Golding have not so far seen fit to reply to my earlier P3R. This is a serious matter: if my criticisms were not well founded I would be grateful to have it explained why the error was mine. Nor can I understand why if the safety of the children is paramount that a line of caution is not being observed in global policy.
[1]Thomas M. Burbacher, Danny D. Shen, Noelle Liberato, Kimberly S. Grant, Elsa Cernichiari, and Thomas Clarkson: 'Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal', http://ehp.niehs.nih.gov/members/2005/7712/7712.pdf
[2] 'Puzzling Circumstances: http://pediatrics.aappublications.org/cgi/eletters/114/3/584#1356
Conflict of Interest:
Parent of an autistic child
Heron and Golding: erroneous premise, anomalous results
Heron and Golding’s paper is based on two false premises, and its results are therefore anomalous. The premises are (a) that they are studying the results of “low doses” of mercury, and (b) that the medium of thimerosal is less pernicious than other familiarly encountered forms of mercury:
“It has been suggested that low doses of ethylmercury might have a similar effect on childhood cognitive development as methylmercury; however, there is little evidence to support this claim. Moreover, ethylmercury is more quickly metabolized and evacuated from the body than methylmercury.”
However, we are told in the very next paragraph:
“Current guidelines on safe exposure to thimerosal have been extrapolated from data on methylmercury and are varied, from 0.1 µg/kg/day of the Environmental Protection Agency in the United States to 0.47 µg/kg/day of the World Health Organization. Before the change to thimerosal-free vaccines, US children could have been exposed to levels as high as 187.5 µg by the time they were 6 months of age, exceeding the Environmental Protection Agency guidelines. In the United Kingdom, the only vaccines that contain thimerosal and have been routinely used in the past 2 decades are whole-cell diphtheria/tetanus/pertussis (wDTP) vaccine or diphtheria-tetanus (DT) vaccine and any combination vaccine containing wDTP or DT. Although the United Kingdom exposure is lower by 6 months, the accelerated United Kingdom primary immunization schedule of 2/3/4 months means that a maximum exposure of 75 µg may be received by 4 months of age. “
So, in fact, these were not “low doses” of Thimerosal at all. Using US CDC growth charts [1,2] I have calculated by how many times approximately UK vaccination practice exceeded the US Environmental Protection Agency's reference dose (RfD) for mercury: "Currently, US EPA uses an RfD of 0.1 micrograms/kg bodyweight/day as an exposure without recognised effects" [3]. Each shot of DPT given at 2,3 and 4 months we are told contained approximately 25 micrograms of mercury, which is 250 times 0.1 microgram. In order to calculate the excess dose you need to divide 250 by the weight of the infant in kilograms. These are the results:
2 months: weight range 3.8-6.4kg: excess dose 40-66 times EPA RfD for mercury
3 months: weight range 4.5-7.4kg: excess dose 35-56 times EPA RfD for mercury
4 months: weight range 5.2-8.3kg: excess dose 30-48 times EPA RfD for mercury
It is impossible to imagine someone deliberately overdosing on a medical product – except to harm themselves – at this level, but this is not a therapeutic substance but a deadly neuro-toxin.
Furthermore, far from thimerosal being an exceptionally benign format for mercury manufacturers' safety advice suggests precisely the opposite. I quote from Merck’s current European safety information:
“Very toxic by inhalation, in contact with skin and if swallowed. Danger of cumulative effects. Very toxic to aquatic organisms, may cause long-term adverse effects in aquatic environment.”
“Keep away from food, drink and animal feeding stuffs. After contact with skin wash immediately with plenty of water. Wear suitable protective clothing. In case of accident or if you feel unwell, seek medical advice immediately (show the label where possible). This material and its container must be disposed of as hazardous waste. Avoid release to the environment.” [4]
A manufacturer’s safety data sheet (MSDS) for thimerosal from Amersham/US Bioscience mentions a long list of symptoms associated with autism:
“Chronic ingestion or excessive dosage may cause numbness, tingling of hands, feet, lips, ataxia, painful joints, constriction of visual fields, impaired hearing, emotional disturbances, spastic movements, incontinence, groaning, shouting, dizziness, lacrimation, hypersalivation, nausea, vomiting, diarrhea and constipation.” [5]
Heron and Golding’s failure to detect any adverse effects is, therefore, quite as surprising as the practice of injecting infants with thimerosal in the first place.
[1] http://www.cdc.gov/nchs/data/nhanes/growthcharts/set1clinical/cj41l018.pdf
[2] http://www.cdc.gov/nchs/data/nhanes/growthcharts/set1clinical/cj41l017.pdf
[3] http://www.epa.gov/mercury/exposure.htm
[4] http://chemdat.merck.de/pls/pi03/web2.search_page2?text=817043&lang=4
[5] http://www.nomercury.org/science/documents/MSDS-Amersham_12-03- 02.pdf