In recent years, thimerosal, an organic mercury com-
pound that is metabolized to ethylmercury and thiosa-
licylate and has been present since the 1930s as a pre-
servative in some vaccines and pharmaceutical products to
prevent bacterial and fungal contamination, has come under
scrutiny. It was determined by the U.S. Food and Drug
Administration (FDA) in 1999 under the recommended
childhood immunization schedule that infants might be ex-
posed to cumulative doses of ethylmercury that exceed
some federal safety guidelines established for exposure to
methylmercury, another form of organic mercury (1).
The hypothesis that exposure to thimerosal-containing
vaccines could be associated with neurodevelopmental dis-
orders is not established and rests on indirect and incom-
plete information, primarily from analogies with methyl-
mercury and levels of maximum mercury exposure from
vaccines given in children. The hypothesis is biologically
possible, but the possible relationship between thimerosal
from vaccines and neurodevelopmental disorders of autism,
attention deficit/hyperactivity disorder (ADHD), and speech
or language delay remains seriously suspect. As of the pres-
ent, there are no peer-reviewed epidemiological studies in
the scientific literature examining the potential association
between thimerosal-containing vaccines and neurodevelop-
mental disorders. Here, we show the first epidemiologic
evidence, based upon tens of millions of doses of vaccine
administered in the United States, that associates increas-
ing thimerosal from vaccines with neurodevelopmental
disorders.
Materials and Methods
In this study, the incidence of neurodevelopmental dis-
orders in a comparative examination between thimerosal-
containing diphtheria, tetanus, and acellular pertussis
(DTaP) and thimerosal-free DTaP vaccines based upon
analysis of the Vaccine Adverse Events Reporting System
(VAERS) database was undertaken using Microsoft Access.
The VAERS database is an epidemiologic database main-
tained by the Centers for Disease Control and Prevention
(CDC) since 1990. All adverse reactions are to be reported
to the VAERS database as required by U.S. law. The CDC
requires written and telephonic confirmation of serious 
ad- verse reactions and follows up on these patients 1 year later.

The FDA inquires into deaths reported to the VAERS da-
tabase by contacting the patient's healthcare provider and
physician. The FDA also continually monitors reports to the
VAERS database to determine whether any vaccine or vac-
cine lot has a higher than expected incidence rate of events.
The VAERS Working Group of the CDC, the FDA, and we
analyze and publish epidemiologic studies based upon
analysis of the VAERS database (2-7).
The neurodevelopmental disorders we analyzed in this
study were autism, mental retardation, and speech disorders.
These categories of adverse reactions were based upon de-
scriptions of adverse reactions by those reporting them and
by defined fields contained in the VAERS database. We
determined the number of male and female reaction reports,
the mean and standard deviation of age in years, and the
mean and standard deviation of onset in days in those ex-
periencing neurodevelopmental disorders after thimerosal-
containing and thimerosal-free DTaP vaccines.
We hypothesize that DTaP vaccines, whether contain-
ing thimerosal or not, should have a similar incidence rate of
adverse reactions. We analyzed DTaP administered by
manufacturer so that we could compare thimerosal-
containing DTaP vaccines administered from 1992 through
2000 against thimerosal-free DTaP vaccines administered
from 1997 through 2000. We used denominators obtained
from the Biological Surveillance Summaries of the CDC to
determine the number of doses of each manufacturer ad-
ministered and, based upon this information, we were able
to calculate incidence rates of adverse reactions after vac-
cination. We are precluded from giving incidence rates, the
number of doses administered, or types of DTaP vaccine
because this information could reveal the identities of the
manufacturers and the CDC claims this information is pro-
prietary between them and the manufacturers (7).
We compared the incidence of adverse reactions after
thimerosal-containing DTaP vaccines against thimerosal-
free DTaP vaccines to determine relative risk (RR), attrib-
utable risk, percent association, and statistical significance.
The RR value was obtained by dividing the incidence rate of
the adverse reaction after thimerosal-containing DTaP vac-
cines by the incidence rate of the adverse reaction after
thimerosal-free DTaP vaccines ([adverse reaction incidence
after thimerosal-containing DTaP vaccines]/[incidence rate
after thimerosal-free DTaP vaccines] _ RR). The attribut-
able risk value was determined by subtracting 1 from the
RR value (RR - 1 _ attributable risk). Percent association
was calculated by dividing the RR value by the RR value
plus 1 and multiplying this computed value by 100 ([RR
/{RR + 1}] × 100). Statistical significance was determined
by using Fisher's Exact Test. Our null hypothesis was that
there would be a statistically similar incidence rate of ad-
verse reactions after thimerosal-containing and thimerosal-
free DTaP vaccines. We assumed that the incidence of ad-
verse reactions after thimerosal-free DTaP vaccines was the
expected rate and the incidence of adverse reactions after
thimerosal-containing DTaP vaccines was the observed
rate. The statistical package contained in Correl's Quattro
Pro was used, and a P value of 0.05 was considered significant.
In addition, to determine if there were potential biases
in our data, we used several controls. We examined the
overall mean and standard deviation of the ages of thimero-
sal-containing and thimerosal-free DTaP vaccine adverse
reactions reported to the VAERS to ensure that both types
of vaccines were administered to similar-aged populations
because different-aged populations may have a difference in
the incidence of neurodevelopmental disorders. The mean
ages of those reporting neurodevelopmental disorders after
vaccination may also help to determine whether successive
doses of thimerosal-containing DTaP vaccines build up con-
centrations of thimerosal to toxic levels, resulting in neu-
rodevelopmental disorders in vaccine recipients. The usual
course of DTaP vaccine in children consists of primary
immunizations administered at 2, 4, and 6 months, followed
up by booster doses at 18 months and at 5 years. We also
examined the incidence rate of acute adverse reactions re-
ported to the VAERS database after thimerosal-containing
and thimerosal-free DTaP vaccines, including deaths, sei-
zures, vasculitis, emergency department (ED) visits, total
reaction reports, and gastroenteritis. This control served
several purposes. First, if differences in manufacturing pro-
cesses, other than the presence of thimerosal were present,
there is a reasonable probability that this might have a sig-
nificant impact on the incidence rate of acute adverse reac-
tions. Second, if biased increased reporting rates of adverse
reactions were present for thimerosal-containing DTaP vac-
cines, this would be reflected in an increased incidence rate
of acute adverse reactions after thimerosal-containing DTaP
vaccines in comparison with thimerosal-free DTaP vac-
cines. The years examined in this study also help to preclude
the possibility of reporting biases based upon popular media
publicity of an association between thimerosal and neurode-
velopmental disorders in recent years because thimerosal-
containing DTaP vaccines were analyzed for much earlier
years (1992-2000) than were thimerosal-free DTaP vac-
cines (1997-2000).

Results
We determined based upon our examination of the
VAERS database that there were a total of 6575 adverse
reaction reports after thimerosal-containing DTaP vaccines
and 1516 adverse reaction reports after thimerosal-free
DTaP vaccines reported to the VAERS database. We found
that thimerosal-containing DTaP vaccines and thimerosal-
free DTaP vaccines were administered to similar-aged
populations. The mean and standard deviations of the ages
were 2.4 ± 3.2 years old and 2.1 ± 2.8 years old, respec-
tively. In Table I, we summarize the number of male and
female reports, mean and standard deviation of age in years,
and mean and standard deviation of onset in days of neu-
rodevelopmental disorders observed after thimerosal-
containing DTaP vaccines and thimerosal-free DTaP vac-rate. 

The statistical package contained in Correl's Quattro
Pro was used, and a P value of 0.05 was considered
significant.
In addition, to determine if there were potential biases
in our data, we used several controls. We examined the
overall mean and standard deviation of the ages of thimero-
sal-containing and thimerosal-free DTaP vaccine adverse
reactions reported to the VAERS to ensure that both types
of vaccines were administered to similar-aged populations
because different-aged populations may have a difference in
the incidence of neurodevelopmental disorders. The mean
ages of those reporting neurodevelopmental disorders after
vaccination may also help to determine whether successive
doses of thimerosal-containing DTaP vaccines build up con-
centrations of thimerosal to toxic levels, resulting in neu-
rodevelopmental disorders in vaccine recipients. The usual
course of DTaP vaccine in children consists of primary
immunizations administered at 2, 4, and 6 months, followed
up by booster doses at 18 months and at 5 years. We also
examined the incidence rate of acute adverse reactions re-
ported to the VAERS database after thimerosal-containing
and thimerosal-free DTaP vaccines, including deaths, sei-
zures, vasculitis, emergency department (ED) visits, total
reaction reports, and gastroenteritis. This control served
several purposes. First, if differences in manufacturing pro-
cesses, other than the presence of thimerosal were present,
there is a reasonable probability that this might have a sig-
nificant impact on the incidence rate of acute adverse reac-
tions. Second, if biased increased reporting rates of adverse
reactions were present for thimerosal-containing DTaP vac-
cines, this would be reflected in an increased incidence rate
of acute adverse reactions after thimerosal-containing DTaP
vaccines in comparison with thimerosal-free DTaP vac-
cines. The years examined in this study also help to preclude
the possibility of reporting biases based upon popular media
publicity of an association between thimerosal and neurode-
velopmental disorders in recent years because thimerosal-
containing DTaP vaccines were analyzed for much earlier
years (1992-2000) than were thimerosal-free DTaP vac-
cines (1997-2000).

Discussion
The results of our analysis were extremely surprising.
We observed statistically significant increases in the inci-
dence rate of neurodevelopmental disorders after thimero-
sal-containing DTaP vaccines in comparison with thimero-
sal-free DTaP vaccines. We observed that the overall mean
age for adverse reactions reported after thimerosal-
containing DTaP vaccines and thimerosal-free DTaP vac-
cines were similar. We found that there were similar inci-
dence rates of acute adverse reactions after thimerosal-
containing DTaP vaccines in comparison with thimerosal-
free DTaP vaccines, indicating that potential differences in
the manufacturing of DTaP vaccines analyzed outside of
thimerosal concentrations or potential population reporting
biases may have had a limited effect on the general reac-
tivity profiles of the DTaP vaccines examined. We also
observed, based upon the mean ages of those developing
neurodevelopmental disorders after thimerosal-containing
DTaP vaccines, that these reactions tended to occur in older
children. This potentially may be explained by the toxic
buildup of mercury from successive doses of thimerosal-
containing DTaP vaccines.
A study performed by Magos et al. (8) in rats compared
the effects of the administration of similar doses of ethyl-
mercury and methylmercury. They found that higher con-
centrations of inorganic mercury in the kidneys and brain
were present in ethylmercury-treated rats compared with
methylmercury-treated rats. They determined that there was
little difference in the neurotoxicites of ethylmercury- and
methylmercury-treated rats when effects on the dorsal root
ganglia or coordination disorders were compared. The au-
thors also determined that microgram quantities of organic
mercury alone in the rat brain were in some cases associated
with neurotoxicity, indicating that the presence of inorganic
mercury was not necessary for neurotoxicity.
The long mean onset times observed in this study for
neurodevelopmental disorders after thimerosal-containing
DTaP vaccines from about 8 to 22 days may be indicative of
the decomposition rates of thimerosal. It has been shown by
Tan and Parkin (9) that thimerosal in vitro decomposes in
the presence of sodium chloride at approximately 4.3% per
day. This means that during the 8- to 22-day temporal pe-
riod of onset observed in this study for neurodevelopmental
disorders, approximately 34.4% to 94.6% of the thimerosal
had decomposed into its derivatives. The authors also report
that it would be expected that the ethylmercury would dis-
play similar complexion and chemical characteristics to
methylmercury. Therefore, considering that sodium chlo-
ride is integrally involved in the functioning of the nervous
system and kidneys, is not potentially surprising that mer-
cury accumulates in these organs, and in the brain, this
accumulation manifests itself in the form of neurodevelop-
mental disorders in some children.
Bernard et al. (10) compared the similar biological ab-
normalities commonly found in autism and the correspond-
ing pathologies arising from mercury exposure. Distinct
similarities were found between autism and mercury expo-
sure in their effects upon biochemistry, the immune system,
the central nervous system structure, neurochemistry, and
neurophysiology. The authors report that mercury toxicity
shows great variability in its effects on the individual, so
that at the same exposure level, some will be affected se-
verely, whereas others will be asymptomatic. They provide
the example of acrodynia, which arose in the early 20th
century, resulting from the use of mercury teething pow-
ders, which afflicted only 1 in 500 to 1000 children given
the same low dose. The authors conclude that due to the
extensive parallels observed between autism and mercury
exposure from thimerosal present in currently used vac-
cines, the likelihood of a causal relationship is great.
We were initially highly skeptical that differences in
the concentrations of thimerosal in vaccines would have any
effect on the incidence rate of neurodevelopmental disor-
ders after childhood immunization. However, the results of
our analysis suggest that children who received an addi-
tional 75 to 100 _g of thimerosal from thimerosal-
containing DTaP vaccines may have an associated increase
in neurodevelopmental disorders based upon analysis of the
VAERS database. Despite showing similarities in the ages
of vaccine recipients between thimerosal-containing and
thimerosal-free DTaP vaccines, similarities in the incidence
rate of acute control reactions after thimerosal-containing
and thimerosal-free DTaP vaccines and a increased age of
onset of neurodevelopmental disorders in those receiving
thimerosal-containing DTaP vaccines, there may be factors
other than thimerosal concentrations that potentially lead to
differences in the incidence rates of neurodevelopmental
disorders observed in this study.
The relative infrequency of neurodevelopmental disor-
ders observed after thimerosal-containing vaccination may
in part reflect the fact that the association between thimero-
sal and neurodevelopmental disorders was not known
among those physicians and therefore was underreported to
the VAERS database, and, in addition, may indicate that
other factors than just thimerosal may effect the incidence
rate of neurodevelopmental disorders. These factors may
include the possibility that mercury is cleared at different
rates, susceptibility among children may change with age or
developmental status, and there may be variation in genetic
composition among different children. It is possible that
these factors may work independently, or more probably,
may work synergistically to produce a neurodevelopmental
response in a susceptible child.
It has also been hypothesized by Wakefield et al. (11,
12) that there may be a specific viral pathogenic mechanism
for a new variant of inflammatory bowel disease among
children with developmental disorders. They have recently
shown a statistical increase in the detection of measles viral
genes in gastrointestinal tissues in children with neurode-
velopmental conditions in comparison with a control popu-
lation (12). Krause et al. (13) have reported that various
immune system abnormalities, including autoimmunity and
defects in different subsets of immune cells, have been re-
ported in children with autistic disorders, suggesting that
immune factors may also play a role in the development of
autism.
In light of the fact that many additional factors may play a 

potential role in the development of neurodevelop- mental 

disorders in children, the observed statistical in-
crease in neurodevelopmental disorders in children receiv-
ing thimerosal-containing DTaP vaccines may reflect a syn-
ergistic effect of multiple factors in a susceptible child. We
recommend that additional studies be conducted to confirm
and extend the results of this study. We suggest that even
though there may be other factors related to the incidence of
neurodevelopmental disorders in children, manufacturers
should consider removing thimerosal from vaccines either
by using another preservative or by producing single dose
vials so that no preservative is necessary, for it is better to
be safe than sorry. Despite these negative findings concern-
ing the preservative thimerosal, vaccination has been and
will continue to be an invaluable asset to control potentially
debilitating and deadly infectious diseases.