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      Pediatric Vaccines and Neurodevelopment: Primate Study Finds No Adverse Behavioral Effects

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      Environmental Health Perspectives
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          Abstract

          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 , 3 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. 1 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. 4 Thimerosal is broken down in the body to thiosalicylate and ethylmercury. 5 The latter is related to neurotoxic methylmercury, and estimates of health risks were based on the assumption—later disproved 6 —that ethyl- and methylmercury have similar toxicity profiles. 5 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. 7 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. 8 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 9 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, 10 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. 4 “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.”

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          The three modern faces of mercury.

          The three modern "faces" of mercury are our perceptions of risk from the exposure of billions of people to methyl mercury in fish, mercury vapor from amalgam tooth fillings, and ethyl mercury in the form of thimerosal added as an antiseptic to widely used vaccines. In this article I review human exposure to and the toxicology of each of these three species of mercury. Mechanisms of action are discussed where possible. Key gaps in our current knowledge are identified from the points of view both of risk assessment and of mechanisms of action.
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            Epidemiology of vaccine hesitancy in the United States.

            Vaccines are among the most effective public health interventions against infectious diseases. However, there is evidence in the United States for parents either delaying or refusing recommended childhood vaccination. Exemptions to school immunization laws and use of alternative schedule from those recommended by the Advisory Committee on Immunization Practices and the American Academy of Pediatrics cannot only increase the risk of children contracting vaccine-preventable diseases but also increases the risk of infecting others who are either too young to be vaccinated, cannot be vaccinated for medical reasons or did not develop a sufficient immunological response to the vaccine. Healthcare providers are cited as the most influential source by parents on vaccine decision-making. Vaccine hesitancy needs to be addressed by healthcare providers and the scientific community by listening to the parental concerns and discussing risks associated with either delaying or refusing vaccines.
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              Safety of vaccines used for routine immunization of U.S. children: a systematic review.

              Concerns about vaccine safety have led some parents to decline recommended vaccination of their children, leading to the resurgence of diseases. Reassurance of vaccine safety remains critical for population health. This study systematically reviewed the literature on the safety of routine vaccines recommended for children in the United States.
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                Author and article information

                Journal
                Environ Health Perspect
                Environ. Health Perspect
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                Environmental Health Perspectives
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                0091-6765
                1552-9924
                01 June 2015
                June 2015
                : 123
                : 6
                : A156
                Affiliations
                [1]Julia R. Barrett, MS, ELS, a Madison, WI–based science writer and editor, is a member of the National Association of Science Writers and the Board of Editors in the Life Sciences.
                Article
                ehp.123-A156
                10.1289/ehp.123-A156
                4455575
                26030194
                343a94cc-0d40-424b-af26-a0e4ea5d94e1

                Publication of EHP lies in the public domain and is therefore without copyright. All text from EHP may be reprinted freely. Use of materials published in EHP should be acknowledged (for example, “Reproduced with permission from Environmental Health Perspectives”); pertinent reference information should be provided for the article from which the material was reproduced. Articles from EHP, especially the News section, may contain photographs or illustrations copyrighted by other commercial organizations or individuals that may not be used without obtaining prior approval from the holder of the copyright.

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                News | Science Selections
                Infectious Disease
                Mental Health
                Mercury
                Neurologic Health

                Public health
                Public health

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