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Ethyl Mercury in vaccines: Was Hilleman right?

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Ethyl Mercury in vaccines: Was Hilleman right? Dr. Y.

By F. Edward Yazbak MD, FAAP

In the last 20 years, my main research interests have been the MMR-Autism connection and the increase in Regressive Autoimmune Autism Spectrum Disorders.

I chose to remain only marginally involved in the Thimerosal-Autism issue because other very well informed researchers were devoting their attention to it. Nevertheless, when something about Thimerosal was worth reporting, I did not hesitate to discuss it. My last series on the subject was published on Vaccination News. [1] [2] [3]

On February 11, 2013 a very interesting article was published On Line[4]. This excellent work by Drs. Dorea, Farina and Rocha titled Toxicity of ethylmercury (and Thimerosal): a comparison with methylmercury” will be published in the Journal of Applied Toxicology where it was accepted on December 11, 2012. Dr. Jose Dorea is a member of the Department of Nutrition, Faculty of Health Sciences at Brasilia University, Dr. Marcelo Farina is with the Department of Biochemistry, Center of Biological Sciences at Brasilia University and Doctor João Rocha is attached to the Department of Chemistry, Center of Natural and Exact Sciences, Federal University of Santa Maria, Brazil.

The authors had no financial or personal conflicts of interest and the research was supported by grants from several of Brazil’s most reputable scientific and educational foundations.


From Hilleman in 1991 …

Maurice R. Hilleman Ph.D. developed over 40 vaccines and published more than 480 original articles during his long career first at Squibb and then since 1957 at Merck. He received many honors including the National Medal of Science from President Reagan, the Distinguished Service Medal for Research, the Special Lifetime Achievement Award from the Children's Vaccine Initiative of the World Health Organization and the Maxwell Finland Award for Scientific Achievement of the National Foundation for Infectious Diseases.

Dr. Hilleman advised several National Institutes of Health committees, the Advisory Committee on Immunization Practices and the World Health Organization over decades. They all had complete faith in his knowledge and expertise except for once: When he wrote in a 1991 Memo that Ethyl and Methyl mercury salts were equally most toxic.

That confidential memo[5] on thimerosal in vaccines by Dr. Hilleman to Dr. Gordon Douglas, Head of Merck’s Vaccine Division at the time, was made public and reported in the LA Times in 2005.[6]

At the time, I was thoroughly impressed by the revelations in the Memo and wrote about them.[7]

The following excerpts from the “Hilleman Memo” are worth repeating:

  1. In Section 5 on page 3, Dr. Hilleman rated the toxicity of mercury compounds as follows:

Most toxic: Methyl and ethyl salts

Intermediate: Mercury vapor

Least toxic: Inorganic mercury salts.

It is important to note that in addition to the three above-mentioned organizations, just about everybody else in the world of vaccination still seems to disagree with Dr. Hilleman, two decades later, regarding the serious toxicity of Thiomersal, an Ethyl Mercury compound used in vaccines. In fact, most “experts” would let you believe that a tuna fish sandwich is more dangerous to a pregnant woman than a couple of mercury-laced vaccines.

  1. In Section 6 e on page 5: Hilleman added: “ For babies:  “The 25 µg of mercury in a single 0.5 ml dose and extrapolated to a 6 lb baby would be 25X the adjusted Swedish daily allowance of 1.0 µg for a baby if that size… If 8 doses of thimerosal-containing vaccine were given in the first 6 months of life (3 DPT, 2 HIB and 3 Hepatitis B) the 200 µg of mercury given, say an average size of 12 lbs would be about 87X the Swedish daily allowance of 2.3 µg of mercury for a baby of that size”

It is important to note that in the United States, the number of recommended pediatric vaccines increased steadily from 1991 to 1999 and so did the potential mercury load that ultimately reached 187.5 µg by age six months. This amount of mercury was much greater than the amount Swedish babies were getting in the first 6 months of life, when the responsible Swedish Health Authorities screamed STOP NOW.


What was more alarming is that before 1999, when we finally limited the mercury load in pediatric vaccines in the United States, the 187.5 µg of mercury were not evenly distributed over the first 180 days of the baby’s life. In fact, at the 2, 4 or 6 month-visits, the infant potentially received 62.5 µg of mercury in seconds [DTP (25µg) + HIB (25µg) + Hepatitis B vaccine (12.5µg)], an enormous amount of mercury in a few seconds for a little baby.  In 1991, we also added the Hepatitis B vaccine to our recommended list in the USA and started administering Thimerosal to newborns before they left the nursery, when vaccination schedules for puppies and kittens only start at age four to six weeks.

The chorus of people and institutions claiming that ethyl mercury in vaccines was no big deal became louder with time and many studies were furtively produced to convince physicians and parents that Thimerosal in vaccines actually was safer than a can of tuna fish. The results of those studies were easily and promptly published in major medical journals in the US and the reports almost always started by: Thimerosal is an ethyl mercury–containing compound that has been used safely for >60 years as a preservative in multidose vials of vaccines…”

Presently, the World Health Organization discusses “Thiomersal in Vaccines” under the heading “Global Vaccine Safety”.[8] The document, extracted from “The Global Advisory Committee on Vaccine Safety: Report of meeting held 6-7 June 2012”[9] , contains two quotes that are worth highlighting:

  1.  “Recently published studies confirm that in all populations studied, including pre-term and low birth-weight babies, the half-life of ethyl mercury in blood is between 3 and 7 days”

2. “Based on the current evidence, GACVS considers that no additional studies of the safety of thiomersal in vaccines are warranted and that available evidence strongly supports the safety of the use of thiomersal as a preservative for inactivated vaccines.”

The first quote intimates that some very intelligent people really believe that mercury goes around and around in the blood of an infant for some 14 days (or at most 30 days) and then goes out in the baby’s stool and urine without any deposition of any kind anywhere in the baby’s head and body.

The second quote echoes one of the conclusions of Meeting 9 of the IOM Immunization Safety Review Committee that was held on February 9, 2004. It is evident that no one paid any attention to that brilliant conclusion then or since and it is likely that no one will start now, just because WHO thinks so. 


On February 13, 2012, the peer-reviewed article by Drs. Dorea, Farina and Rocha was listed on PubMed       [PMID 23401210]. Just for clarification PubMed is really, The United States National Library of Medicine, National Institutes of Health.

In the United States, no more prestigious listing exists.

In the PubMed publication abstract, the authors wrote:

“Ethylmercury (etHg) is derived from the metabolism of thimerosal (o-carboxyphenyl-thio-ethyl-sodium salt), which is the most widely used form of organic mercury. Because of its application as a vaccine preservative, almost every human and animal (domestic and farmed) that has been immunized with thimerosal-containing vaccines has been exposed to etHg. Although methylmercury (meHg) is considered a hazardous substance that is to be avoided even at small levels when consumed in foods such as seafood and rice (in Asia), the World Health Organization considers small doses of thimerosal safe regardless of multiple/repetitive exposures to vaccines that are predominantly taken during pregnancy or infancy. We have reviewed in vitro and in vivo studies that compare the toxicological parameters among etHg and other forms of mercury (predominantly meHg) to assess their relative toxicities and potential to cause cumulative insults. In vitro studies comparing etHg with meHg demonstrate equivalent measured outcomes for cardiovascular, neural and immune cells. However, under in vivo conditions, evidence indicates a distinct toxicokinetic profile between meHg and etHg, favoring a shorter blood half-life, attendant compartment distribution and the elimination of etHg compared with meHg. EtHg's toxicity profile is different from that of meHg, leading to different exposure and toxicity risks. Therefore, in real-life scenarios, a simultaneous exposure to both etHg and meHg might result in enhanced neurotoxic effects in developing mammals. However, our knowledge on this subject is still incomplete, and studies are required to address the predictability of the additive or synergic toxicological effects of etHg and meHg (or other neurotoxicants). Copyright © 2013 John Wiley & Sons, Ltd.”

In their abstract, Dorea et al brought up several important points that seem to have escaped the attention of WHO and the “Thimerosal Booster Club” in the USA:

  1. That a simultaneous exposure to both ethyl and methyl mercury might result in “enhanced” neurotoxicity in developing mammals
  2. That ethyl and methyl mercury had “equivalent measured outcomes for cardiovascular, neural and immune cells” in vitro studies
  3. That ethyl mercury and methyl mercury have different toxicity profiles and therefore have different toxicity risks

The full article is 12-page long. It includes several lists and graphs and … some 140 references.

Under Concluding Remarks, the authors summarized their research findings as follows:

The differences in mercury metabolism in different organs largely result from the binding capacities of mercurys chemical forms and from the stability of carbonmercury linkages in organic mercurials.

The neurotoxicity of etHg is similar to meHg in most in vitro systems, but differences in the kinetics between these two compounds display differences in tested outcomes. However, an immunotoxicity is more pronounced and more common for thimerosal etHg.

The differences in the toxicities of these two contemporary and relevant forms of Hg can be explained by the faster degradation (spontaneous or enzymatic) of etHg when compared with meHg and Hg(II). Because the targets of these mercurials do not completely overlap, a simultaneous exposure to meHg and etHg can have unpredictable additive and synergistic effects on developing and mature humans.

Age and type of exposure, route, and attendant differences in mercury chemistry make toxicological comparisons with etHg and meHg useful in understanding the complexity of mercury metabolism but not sufficient to establish safety recommendations.

Existing animal models demonstrate that etHg is less neurotoxic than meHg, but we still require adequate models to demonstrate whether repeated relevant doses of etHg in combination

with different meHg background exposures have consequences on fetuses and infants.

EtHg and meHg are different compounds and lead to different exposure levels and different toxicity risks.

Although few animal models have compared the toxicity of etHg to meHg (at high doses), a few human observational studies have indicated that when simultaneous low doses of etHg and meHg exposure occur during the perinatal period, there are weak associations with neurodevelopmental outcomes. Consequently, further detailed studies with low levels of simultaneous exposure to meHg and etHg are required to establish the hypothetical no-observed adverse effect level (NOAEL) in experimental models using different endpoints of toxicity (from biochemical to neurobehavioral determinations). Most importantly, large epidemiological studies are also required to ascertain whether simultaneous

exposure to alkylmercurials can have more than additive long-lasting neurotoxicological effects on children.”


Even though the authors were careful and understated, their research of a serious Global Health problem was remarkable.

Of their many important findings, the following three deserve highlighting:

  • The immunotoxicity of Thimerosal Ethyl Mercury is more pronounced and more common than that  of Methyl Mercury
  • Because Ethyl Mercury and Methyl Mercury compounds are different, they lead to different toxicity risks
  • And lastly, because the targets of Ethyl and Methyl Mercury compounds do not completely overlap, a simultaneous exposure to both compounds can have additive and synergistic effects on developing humans (and even on mature ones)


After 22 years, the answer to the question in the title is YES and Dr. Maurice Hilleman was right: “Ethyl and Methyl Mercury compounds are most toxic”

Hopefully the WHO decision-makers, the members of the involved committees of the IOM and the Special Masters of our Vaccine Injury Compensation Program will notice.

It is still not too late!


F. Edward Yazbak MD, FAAP, Falmouth, Massachusetts 02540