Note from Health Lover, Ilena Rosenthal:
http://ilenarose.blogspot.com
http://www.huffingtonpost.com/david-kirby/cdc-vaccine-study-used-fl_b_108462.html
(NOTE: My original post on this topic mischaracterized the 2003 CDC
vaccine investigation as an "Ecological Study," which it was not. I am
reposting this piece to reflect that information accurately, but also
to point out that many of the weaknesses identified in the CDC's data
and methods apply to the published 2003 "retrospective cohort" study,
as much as they do to any future "ecological" ones. I regret and
apologize for the error.)
A new re****t that CDC Director Dr. Julie Gerberding has delivered to
the powerful House Appropriations Committee casts new light - and new
doubt - on the data and methodology that the CDC used in its landmark
2003 study that found no link between mercury in vaccines and autism,
ADHD, speech delay or tics.
Gerberding was responding to a re****t from the National Institute of
Environmental Health Sciences (NIEHS), which evaluated the strengths
and weaknesses of the CDC's vaccine database, and showed how the
weaknesses, in particular, would have to be addressed in conducting
further studies of thimerosal and autism.
These weaknesses included: Uncertainties in case ascertainment,
heterogeneity of business practices within and across HMOs in the
database and their systematic changes over time, misclassification of
exposure status, and the inability to control for tem****al changes in
awareness, diagnostic practices and potential confounding factors.
Many of these weaknesses should be taken into account when moving into
the future, but they also apply to CDC studies that have been done in
the past, including the methodology that was employed in the CDC's
flag****p thimerosal safety study of 2003.
To begin with, the NIEHS panel had "identified several areas of
weaknesses that when taken together reduce the usefulness of the
project for conducting an ecologic study design to address the
potential association between exposure to thimerosal and the risk of
autism."
Ecological studies are large, epidemiological analyses of risks and
trends using data from large populations without making efforts to
link outcomes to actual individual patients. The 2003 CDC study was
not, strictly speaking, an "ecological study," but rather a
"retrospective cohort study."
CDC researchers did go back and review some of the charts of the
children diagnosed with the outcomes under study - though this
accounted for less than 1% of all children enrolled in the study.
Dr. Gerberding said in her letter that thimerosal studies done by the
CDC have not been "ecological," because they utilize "medical chart
reviews, neurological *****sments, and parent interviews." But in the
2003 study, chart reviews were not done on 99% of the study
population, and no *****sments or interviews were conducted to make
sure that the diagnoses were accurate.
The NIEHS re****t was largely focused on the feasibility of conducting
an ecological study of the database, but many of the weaknesses
identified are also applicable to the 2003 CDC study of the Vaccine
Safety Datalink (VSD) which contains the records of hundreds of
thousands of HMO patients.
In that investigation, CDC officials conducted at least five separate
analyses of the data over a four-year period from 1999-2003. The first
analysis showed that children exposed to the most thimerosal by one
month of age had extremely high relative risks for a number of
outcomes, compared with children who got little or no mercury: The
relative risk for ADHD was 8.29 times higher; for autism, it was 7.62
times higher; ADD, 6.38 times higher; tics, 5.65 times; and speech and
language delays were 2.09 more likely among kids who got the most
mercury.
Over time, however, all of these risks declined into statistical
insignificance, statistical inconsistency or else outright oblivion:
The relative risk for autism plummeted from 7.62 in the first
analysis, to 2.48 in the second version, to 1.69 in the third round,
to 1.52 in the fourth, and down to nothing at all in the fifth, final,
and published analysis printed in the Journal Pediatrics in November
of 2003.
Vaccine officials attributed the steady drop to the elimination of
"statistical noise" from the data through due diligence and the
endeavor for excellence in governmental statistical analysis.
Indeed, the VSD study was the main pillar of a hugely influential 2004
re****t by the Institute of Medicine, which also concluded that there
was no evidence of link between mercury, vaccines and autism.
To this day, public health officials routinely point to five "large
epidemiological studies" representing the "highest quality science,"
none of which found any link to thimerosal.
In fact, the American VSD study has long been held up as the best and
brightest of them all (the others were in Sweden, the UK, and two in
Denmark, which WERE ecological studies, and presumably subject to some
of the same weaknesses identified by the NIEHS). This reputation has
stuck in the minds of medicine and the media.
Curiously though, even the study's lead author -- Dr. Thomas
Verstraeten, an employee of vaccine maker GlaxoSmithKline -- protested
that the VSD study "found no evidence against an association, as a
negative study would. In fact, he said that additional study was
needed, which "is the conclusion to which a neutral study must come."
That's when Congress stepped in.
In 2005, a group of Senators and Representatives headed by Sen. Joe
Lieberman wrote to the NIEHS (an agency of the National Institutes of
Health) saying that many parents no longer trusted the CDC to conduct
independent minded studies of its own vaccine program. Lieberman et al
asked NIEHS to review the CDC's work on the vaccine database and
re****t back with critiques and suggestions for future investigations.
Among the official tasks given to the NIEHS panel were to "Identify
the strengths and weaknesses of the VSD for evaluating the possible
association between exposures to thimerosal-containing vaccines and
AD/ASD," and had nothing to do with "ecological studies."
In her letter to the House Appropriations Committee, the CDC Director
responded directly to many -- though not all -- of the most im****tant
critiques and recommendations contained in the NIEHS panel re****t. The
weaknesses she noted apply to Verstraeten as much as they do to any
future studies, ecological or otherwise.
For example, the NIEHS re****t had said the VSD data failed to account
for other mercury exposures, including maternal sources from flu shots
and immune globulin, as well as mercury in food and the environment.
"CDC acknowledges this concern and recognizes this limitation," the
Gerberding reply says.
The NIEHS also questioned why CDC investigators eliminated 25% of the
study population for a variety of reasons, even though this
represented, "a susceptible population whose removal from the analysis
might unintentionally reduce the ability to detect an effect of
thimerosal." This strict entry criteria would likely lead to an
"under-ascertainment" of autism cases, the NIEHS re****ted. Again, this
would have been an issue in the Verstraeten data.
"CDC concurs," Gerberding wrote, again noting that VSD data are "not
appropriate for studying this vaccine safety topic. The data are
intended for administrative purposes and may not be predictive of the
outcomes studied."
Another serious problem is that the HMOs have changed the way they
tracked and recorded autism diagnoses over time. Gerberding said this
would be a problem going into the future, but did not mention that the
same principal might apply to past studies.
I hope everyone will read these do***ents, including the
recommendations to make the VSD better, and the CDC's agreement with
all of the suggestions. Hopefully, this data can still be used in some
effective way.
As questionable as the US thimerosal methodology was, "it was an
improvement on other studies, including the two ecological studies in
Denmark, both of which had serious weaknesses in their designs," Dr.
Irva Hertz-Picciotto, Professor of Public Health at UC Davis Medical
School and Chair of the NIEHS panel, told re****ter Dan Olmsted at UPI.
That leaves little for the CDC to go on in terms of proving that
thimerosal and autism are not associated in any way.
Yes, there is always the study of disability services data from
California -- which seem to be rising among the youngest cohorts of
kids, who presumably received little or no mercury because thimerosal
was largely removed from childhood shots.
But California is an "ecological study" as well, with problems of its
own.
"Although (this) information is often used by media and research
entities to develop statistics and draw conclusions, some of these
findings may misrepresent the quarterly figures," cautions the website
of the California Department of Developmental Services (DDS).
"Increases in the number of persons re****ted from one quarter to the
next do not necessarily represent persons who are new to the DDS
system."
Even the CDC admits that "there are several limitations" with linking
a VSD study design with the California data, Gerberding wrote to
Congress, because, among other things, California only counts "persons
who were referred to and/or voluntarily entered" the disability
system."
It will be interesting to see how the House Committee -- and the
mainstream media -- react to this re****t by, which does seem to want
to conduct the best vaccine-autism science possible (see Gerberding's
replies to NIEHS recommendations for improving the VSD: CDC officials
are currently conducting in- depth follow up studies with VSD
patients).
POSTSCRIPT:
This revised piece does raise two new questions, I think:
1) If the VSD is not necessarily appropriate to help determine the
effect of reducing mercury levels in vaccines, are taxpayers getting
their money's worth?
2) If studies done in Denmark, Sweden and California were also
"ecological" in nature, are they subject to some of the same
weaknesses and limitations?
|
