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Thimerosal, MMR vaccine and autism: moving toward a climax
Controversy surrounding court cases may broaden public distrust of the vaccine establishment within anti-vaccination groups and beyond.
In this issue of Infectious Disease News is a story updating readers on the first alleged case of autism caused by either thimerosal, measles-mumps-rubella vaccine or both to be tried in the U.S. Court of Federal Claims under the Vaccine Injury Compensation Program [First autism-vaccine test case goes to court under Vaccine Injury Compensation Program].
This topic is certainly of greater immediate interest to pediatricians than to internists. The implications of these proceedings are so broad, however, that I will use this space again, as I have done several times in the past, to update readers on the issues.
There are close to 5,000 cases of alleged injury resulting in autism before the court; these have been lumped together in the “Omnibus Autism Bundle.” Nine “test” cases will be heard in detail by three Special Masters (judges), each exemplifying one of the three theories of injury: thimerosal alone causes autism; MMR vaccine causes autism; or thimerosal and MMR vaccine, acting in concert, result in autism. It might well be next year before these proceedings are concluded and results made public.
Background
It is instructive to be aware of how we arrived at this point in the United States. Thimerosal is ethyl mercury, a potent microbicide; it has been used for decades as a preservative in multidose vials of many killed vaccines all over the world in a concentration no greater than 0.01%. It has never been linked to, or even suspected of, being a health hazard when used in that manner.
However, a related, but not identical, compound — methyl mercury — became recognized during the past five decades as a widespread environmental toxic pollutant. It is also well known to be a human health hazard, the principal target being the central nervous system.
Three separate agencies of the federal government have independently developed standards for minimum allowable doses of methyl mercury ingestion, and they differ by as much as four- to fivefold. All of them, however, are based on the lowest observed levels at which symptoms start to develop in experimental animals and then divided by a “safety factor” of 10. It is important to note that no such standards exist for ethyl mercury, the active agent in thimerosal.
In the 1990s, as the pediatric immunization schedule expanded substantially with the addition of new vaccines (eg, hepatitis B and Haemophilus influenzae type b), the total ethyl mercury exposure in the first six months of life began to approach or even exceed (depending on which agency’s standards were being used) the recommended limits of methyl mercury exposure.
In the late 1990s, as part of a global effort to get mercury out of vaccines in general, a decision was made, rather precipitously and with little scientific debate, to eliminate the use of thimerosal as a preservative in vaccines in the United States. The explanation to the public that this was being done simply as a safety measure was hardly reassuring and led to further concern that the federal government and the vaccine manufacturers knew a good deal more than they were telling the public about the damage being caused by ethyl mercury in vaccines.
It was thus all too easy for the vaccine critics to make the short leap from concern about mercury to “thimerosal causes autism.” The reason advanced that it does not result in autism in all thimerosal recipients is that it happens only in those few infants who have some as yet undefined “genetic predisposition.”
Science on trial?
None of this is about science. The science has been and will continue to be heard in these proceedings. There are by this time a substantial number of large population-based trials, both in this country and abroad, that demonstrate no association at all of autism with administration of thimerosal, MMR vaccine or hepatitis B vaccine, either alone or in combination.
The Institute of Medicine concluded several years ago that there was no evidence favoring an association of pediatric immunization with the development of autism. Indeed, if thimerosal were the culprit, one would have expected the incidence of autism to decline as thimerosal was removed from pediatric vaccines; no such decline has been observed. Although it is logically impossible to prove a universal negative, there is an abundance of epidemiologic evidence that has not found credible evidence of such a link.
And yet, in a certain sense, science is on trial here. The legal standard used to decide these cases is not “preponderance of evidence,” but rather simply “plausibility.” It is not an evidence-based standard. In the larger sense, the whole public health immunization enterprise is on trial.
There are enormous global implications in the outcome of these proceedings. The WHO Global Immunization Program uses thimerosal-containing tetanus-diptheria-pertussis vaccines, as they have done for many years, virtually throughout the developing countries. If the vaccine court in the United States decides that thimerosal does, or even may, cause autism, the developing countries may well protest the use of thimerosal-containing vaccines by WHO. Removing thimerosal, however, would result in far more expensive vaccines that may well be simply unaffordable for many countries. Thus, global public health is also threatened.
In the United Kingdom, and to a lesser extent in Canada, the concern seems to be focused much more on MMR vaccine than on thimerosal. The so-called Wakefield paper, which purported to show that measles virus (from MMR vaccine) persisted in lymphoid tissue in the small bowel and resulted in developmental disorders such as autism, has by this time been thoroughly discredited; however, the damage had been done as far as public acceptance of MMR vaccine was concerned.
MMR vaccination rates in the United Kingdom at times sank to 75%, and localized outbreaks of all three diseases continue to occur. It is believed, for example, that the present outbreak of mumps in Nova Scotia resulted from an importation from the United Kingdom. According to one report, the British physician governing body is conducting hearings into whether Wakefield and his colleagues were unethical and/or dishonest in the conduct of their research — hearings that could possibly result in loss of medical licensure.
So although a climax seems to be approaching, both in the United States and the United Kingdom, no possible result will end the controversy. No matter what the result, public distrust of the vaccine establishment will likely continue to fester within anti-vaccination groups and beyond.