The American Academy of Pediatrics
provides the following information for clinicians who may be aware of recent
press surrounding an article that claims to show a correlation between
thimerosal and autism.1 This paper uses data from the Vaccine Adverse Event
Reporting System (VAERS) inappropriately and contains numerous conceptual and
scientific flaws, omissions of fact, inaccuracies, and misstatements.
The most important weakness of the article
is the reliance on VAERS data to draw conclusions about adverse event
associations or causality. VAERS is a passive surveillance system for reporting
possible vaccine adverse events that depends on health care professionals,
patients, and others to file reports. Health effects reported to VAERS as being
associated with vaccines may represent true adverse events, coincidental
occurrences, or mistakes in filing. Inherent limits of VAERS include incomplete
reporting, lack of verification of diagnoses, and lack of data on people who
were immunized and did not report problems. Data from VAERS are useful for
hypothesis generation (raising questions) but should not be used for research
aimed at determining whether vaccines cause certain health problems (hypothesis
proving), as was done in the article by Geier and Geier. For example, VAERS
worked well to quickly alert investigators to the possibility of intussusception
after rotavirus immunization but could not prove the association. Proof required
numerous controlled studies to document the nature and frequency of this
association.
The original concern regarding thimerosal
in vaccines was sparked not by any trends identified in the VAERS system after
70 years of experience with thimerosal use as a vaccine preservative but by
theoretic concerns about total exposures infants might receive from all mercury
sources in the environment, including vaccines. Research to date involving
refined, controlled studies in large populations of patients has failed to
demonstrate any association between vaccines that may have used thimerosal as a
preservative and neurodevelopmental disorders including autism. The authors
failed to acknowledge the inherent limitations of the VAERS database when
drawing conclusions of adverse event associations contained in this report and
their other publications. They are equally unclear as to how their data were
generated, thus preventing accurate review of their methods and replication of
their outcomes.
Other flaws in the article include the
following: _ The law relating to VAERS reporting is misstated. Most
VAERS-reported conditions fall into a category in which voluntary and passive,
not mandatory or required, events after immunization are recorded. Only a
specific set of more severe adverse events are specified as mandatory under the
Vaccine Injury Table, and even then, reporting is inconsistent.
_ Conclusions of the 2001 Institute of
Medicine Immunization Safety Review Committee report2 as to what constitutes
maximal permissible dose exposures to mercury are misinterpreted, and misleading
statements are made concerning federal safety guidelines for mercury exposure
levels that might be expected to cause harm.
_ The authors fail to depict accurately
the differences between pharmacokinetics of and exposure to methylmercury (found
in contaminated
food) and ethylmercury (found in thimerosal) and
make unsubstantiated assumptions about the risks of the route of exposure
(ingested versus injected).
_ Adult heart disease is included as a
possible thimerosal-related condition, although heart arrest reports in very
young children are used in the analysis. Heart arrest in very young children (a
common term used on pediatric death certificates and often unrelated to the
actual cause of
death) has nothing to do with adult coronary
heart disease. The authors implication that thimerosal in vaccines is a cause
of acute cardiotoxic events is unfounded in any scientific or clinical reports
and represents a misuse of the terminology found in VAERS reports.
_ The authors fail to reveal how
thimerosal exposure was calculateda critical omission, because much of the data
required to estimate mercury exposure are not available in VAERS reports. The
authors stated estimates of exposure attributable to diphtheria, tetanus, and
pertussis combination vaccines (DTaP or DTwP) do not add up. Some DTaP vaccines
never contained thimerosal as a preservative, and any child may have received 1
or more DTaP doses, which would have resulted in no ethylmercury exposure.
_ The authors claim to have analyzed data
from biologic surveillance summaries by manufacturers, although data regarding
specific manufacturers (some of which incorporated thimerosal as a preservative
and some of which did not) and age and year of birth of vaccine recipients are
not available in the publication cited. Data as to the number of patients
receiving vaccines with thimerosal plus the number of doses of vaccine actually
received by patients versus total doses of vaccine manufactured cannot be
derived from biologic surveillance summaries, making the authors claims for
baselines of actual vaccine use untenable.
_ Calculations for incidence rates and
relative risk, which require information (age or year of birth) that is not
available from biologic surveillance data, are not shown.
_ An appropriate comparison is not made
between thimerosal exposure and no thimerosal exposure, which is not possible
using VAERS data, because one cannot be sure whether a child received a
thimerosal-containing vaccine at any point before the event for which the VAERS
report was created. Depending on the manufacturer, many of the children listed
in VAERS reports could have received all vaccines that were free of thimerosal.
_ Statistical methodology for calculating
the relative risk and correlation coefficients is not stated.
_ The authors claim to have performed
their own analysis of a Vaccine Safety Datalink (VSD) thimerosal screening study
(reference 17 in Geier and Geier), although the raw data needed to perform an
independent analysis are not available in the document cited. (Note: neither the
original preliminary VSD study of thimerosal and neurodevelopmental disorders
nor any of the follow-up expanded studies identified a signal indicating any
association between thimerosal and autism.) _ The authors claim that data for
thimerosal exposure and autism risk follow an exponential distribution, although
none of the thimerosal exposure categories had a significantly increased risk of
autism. The figures used are confusing and not supported by an adequate
explanation as to how they were constructed. Comparing the occurrence of late
onset, chronic conditions like autism by using acute vaccine reactions like
fever, pain, and vomiting (presumably attributable to other vaccine
components) as controls makes no sense as a
measure of relative adverse event rates.
_ When comparing early (1984-1985) to late
(1990-1994) birth cohorts, the authors make arbitrary and unlikely assumptions
of possible thimerosal exposure for both groups that are contrary to when
thimerosal vaccines were introduced and what their expected pattern of use in
the private and public sector was. The average level of thimerosal exposure
claimed by the authors is not realistic.
_ The authors claim high correlation
coefficients for thimerosal with certain neurologic disabilities without
describing the statistical methods used, which makes the results highly
unreliable.
_ The authors fail to note that a recently
published review by Nelson and Bauman3 casts doubt on the biologic plausibility
of symptom similarities between mercury poisoning and autism.
_ The authors claim falsely that children
in the United States in 2003 may be exposed to higher levels of mercury from
thimerosal contained in childhood immunizations than any time in the past, when
in fact, all routinely recommended infant vaccines currently sold in the United
States are free of thimerosal as a preservative and have been for more than 2
years (www.fda.gov/cber/vaccine/thimerosal.htm#1).
No scientific data link thimerosal used as
a preservative in vaccines with any pediatric neurologic disorder, including
autism. Despite this, the Centers for Disease Control and Prevention, American
Academy of Pediatrics, National Institutes of Health, and US Public Health
Service have continued to investigate this issue to put theoretic concerns about
this mercury-containing compound to rest. Thimerosal continues to be used widely
as a vaccine preservative in many other parts of the world where economics and
sanitation concerns mandate an effective means to safeguard vaccines from
contamination when stored in bulk in multidose vials. Any scientific article
that can prove a thimerosal link to significant adverse events in children must
be published in respected and widely read journals because of the great general
interest today in vaccine safety. These journals can be expected to apply the
highest standards of critical peer review to the results of any research that
purports the existence of these associations and claims of causality.
The American Academy of Pediatrics
negative response to the Geier & Geier article linking thimerosal in DTPs to
autism and speech disorders is not surprising given the AAP's central role in
promoting childhood immunizations. In an effort to clear thimerosal in vaccines
of any role in neurodevelopmental disorders when studies establishing
thimerosal's safety do not exist, the AAP relies on a number of distortions and
inaccuracies. Many points made in the response would need to be answered by Drs.
Geier & Geier, but several should be addressed immediately.
(a) The AAP paper asserts that the VAERS
data base is only valid for hypothesis generation. If so, then at the very least
the Geiers' paper should be viewed by the AAP as advancing a strong hypothesis.
If they were truly unbiased, the AAP should be asking for follow up controlled
trials to be initiated immediately to see if the hypothesis is true rather than
dismissing it out of hand. Moreover, it is the AAP's own members who do such a
poor job of reporting adverse events to VAERS. If the AAP were sincerely
interested in vaccine safety investigations, they could improve the quality of
VAERS dramatically by requiring their members to report.
(b) The AAP paper says (Note: neither the
original preliminary VSD study of thimerosal and neurodevelopmental disorders
nor any of the follow-up expanded studies identified a signal indicating any
association between thimerosal and autism.) This is simply not true. The first
VSD analysis gave a relative risk for autism of 2.48. The VSD thimerosal study
protocol, written by the authors before results were in, clearly states that a
RR higher than 2.0, even if not statistically significant, constitutes a signal
which should be investigated in a phase II study that would confirm or not
confirm the association. This phase II study has never been initiated by CDC. In
fact, there are no expanded VSD studies - the CDC merely divided up the data
sets from the HMOs studied, which resulted in insufficient numbers of cases to
reach statistical significance for any given HMO.
(c) The AAP says: Research to date
involving refined, controlled studies in large populations of patients has
failed to demonstrate any association between vaccines that may have used
thimerosal as a preservative and neurodevelopmental disorders including autism.
Again, this statement is false. There are no large controlled studies which have
investigated thimerosal and developmental disorders. (The VSD analysis was a
retrospective cohort study.)
(d) The AAP critique explains that any
study on this topic must be published in respected and widely read journals
because of the great general interest today in vaccine safety. Yet they go on
to cite the recently published review by Nelson and Bauman in their own
newsletter which they assert casts doubt on the biologic plausibility of
symptom similarities between mercury poisoning and autism. The Nelson and
Bauman commentary was an invited article and was not subjected to peer review.
(e) In what appears to be a Freudian slip,
the AAP says the Centers for Disease Control and Prevention, American Academy
of Pediatrics, National Institutes of Health, and US Public Health Service have
continued to investigate this issue to put theoretic concerns about this
mercury-containing compound to rest. It is hoped that these agencies would be
investigating this issue to find out the truth, not to lay the issue to rest.
The fact that these agencies have every reason to conduct research with a hidden
agenda is the reason why investigations of thimerosal and vaccine safety need to
be conducted by unbiased researchers with no conflicts of interest.
(f) The AAP does admit that Comparing the
occurrence of late onset, chronic conditions like autism by using acute vaccine
reactions like fever, pain, and vomiting...as controls makes no sense as a
measure of relative adverse event rates. Given that late onset, chronic
conditions are very different from immediate, acute ones, perhaps the AAP could
voice support for requiring vaccine safety trials by manufacturers to extend
beyond the current practice of 60 days and to include monitoring of chronic
conditions. Then parents might have more confidence in the never-ending
assurances by the public health authorities that vaccines are indeed safe.
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as providing medical or legal advice. The decision whether or not to vaccinate
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"A foolish faith in authority is the worst enemy of truth."
-- Albert Einstein, letter to a friend, 1901
"I know of no safe depository of the ultimate powers of the society but the people themselves, and if we think them not enlightened enough to exercise control with a wholesome discretion, the remedy is not to take it from them, but to inform their discretion by education."
-- Thomas Jefferson, letter to William C. Jarvis, September 28, 1820
"What's the point of vaccination if it doesn't protect you from the unvaccinated?"
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