Vaccination has successfully reduced the incidence and prevalence of many infectious
diseases. However, in the absence of outbreaks, a perception has developed that vaccinations
themselves may present a greater risk than the diseases they prevent.
1
Although evaluations of the existing data have failed to identify links between vaccination
and negative health outcomes,
2
,
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there remain some public concerns about a potential link with autism spectrum disorders.
These concerns are primarily due to the expansion of the infant immunization schedule
and exposure to thimerosal, once used as a preservative in some vaccines.
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In this issue of EHP, the authors of a detailed exploration of neurodevelopment, learning,
and social behavior in macaques report no adverse developmental and behavioral effects
associated with the full pediatric vaccine schedule.
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Thimerosal is broken down in the body to thiosalicylate and ethylmercury.
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The latter is related to neurotoxic methylmercury, and estimates of health risks were
based on the assumption—later disproved
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—that ethyl- and methylmercury have similar toxicity profiles.
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With the exception of inactivated influenza vaccine, thimerosal has been removed from
or reduced to trace amounts (≤ 1 µg/dose) in all vaccines routinely recommended for
children under 6 years of age.
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Nevertheless, concerns persist, and vaccination rates have declined in some populations,
with subsequent reemergence of diseases such as the recent outbreak of measles originating
in California.
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Results from a study of macaque neurodevelopment bolster the body of evidence that
routine pediatric vaccines are safe for human children.
© TEK IMAGE
“The main impetus behind the study was that while individual pediatric vaccines underwent
the required clinical safety testing, the cumulative exposure to multiple thimerosal-containing
vaccines had not been examined,” says study coauthor Laura Hewitson, director of research
at the Johnson Center for Child Health and Development in Austin, Texas. The current
study scrutinized neurodevelopment in macaques, whose nervous system development follows
a similar trajectory to humans. Macaques also have learning and memory processes and
social interactions that mimic those observed in humans.
Seventy-nine macaques were assigned to six vaccination groups: 1) controls—animals
received saline injections only; 2) MMR—animals received only the measles, mumps,
and rubella vaccine, which has never contained thimerosal; 3) TCV—animals received
all the thimerosal-containing vaccines administered routinely to children in the 1990s
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but no MMR vaccine; 4) 1990s Pediatric—animals received all pediatric vaccines administered
in the 1990s, including TCVs and MMR, on the schedule recommended for children; 5)
1990s Primate—animals received all pediatric vaccines administered in the 1990s on
an accelerated schedule to match macaque development; and 6) 2008—animals received
the expanded immunization schedule in place as of 2008,
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which is similar to the current schedule.
Between birth and 12 months of age, animals were tested for neonatal reflexes, object
permanence (a measure of early memory development), discrimination learning strategies
(ability to respond differently to different stimuli), and social behaviors. Animals
also were systematically observed and rated for their level and characteristics of
exploration, social withdrawal, play behavior, and aggression.
The investigators found that animals within each dosing group showed similar development
of neonatal reflexes and object permanence. There was a slight indication that the
TCV and 1990s Primate groups performed better in some learning tests, and some of
the experimental groups demonstrated fewer negative social behaviors than the control
group. However negative behaviors were rarely observed in any group, and learning
and social development overall appeared normal.
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“Some analyses suggested a beneficial effect of the thimerosal exposure,” says Neal
Halsey, director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg
School of Public Health. However, says Halsey (who was not involved in the study),
there currently is no plausible biologic mechanism to explain those associations,
which makes it more likely they were found by chance.
“Most importantly, in terms of the behavioral data, we saw virtually no negative behaviors
across all study groups,” says Hewitson. “That speaks to the hypothesis of thimerosal
directly affecting behavior,” she says. “The fact that we didn’t see an increase in
negative behaviors is reassuring.”