Immunization Safety Review:Vaccines and Autism

Marie McCormickChair, Immunization Safety Review Committee

Presentation to NVAC June 2004

Issue Under Review

Hypothesized association between vaccines and autism, specifically:

• MMR vaccine and autism • Thimerosal-containing vaccines and autism

• The committee did not focus on other neurodevelopmental disorders

Why the committee focused on this topic

• Requested by the Interagency Vaccine Group to reexamine this issue in its eighth and final report. – The committee issued two previous reports in 2001

examining MMR and autism and thimerosal-containing vaccines and neurodevelopmental disorders.

• Updated statement from the committee was warranted because significant new data have emerged, and because the topic remains of such considerable controversy.

Scientific Assessment: Causality Conclusions

The evidence favors rejection of a causal relation between both MMR vaccine and thimerosal-containing vaccines and autism.

Scientific Assessment: Causality Conclusions

Evidence for MMR and autism finding: • 14 large, well-designed epidemiological studies

consistently showed no association between the MMR vaccine and autism.

This finding is consistent with 2001 report on MMR and autism

Scientific Assessment: Causality Conclusions

Evidence for thimerosal and autism finding: • 5 large, well-designed epidemiological studies in different countries

provided significant evidence of no association between TCVs and autism.

2001 IOM report concluded evidence was inadequate to accept or reject relationship between TCVs and NDDs. Why different now? – Only one unpublished epi study available then. Significantly more

research available now.– That report focused on broader set of neurodevelopmental outcomes.

A potential biological mechanism exists for those outcomes by way of analogy with methyl mercury, but not for autism

Scientific Assessment:Biological Mechanisms

Potential biological mechanisms put forth as possible explanations for how vaccines might cause autism:

– The release of chemicals into the brain due to disruption of intestinal function by the MMR vaccine

– Triggering of abnormalities in the immune system that are indicative of vaccine-induced damage to the CNS

– Increased accumulation of and decreased excretion of mercury from the brains of a subgroup of children

– The effects of thimerosal on a variety of biochemical pathways

Scientific Assessment:Biological Mechanisms

• Evidence comes from in vitro experimental systems, clinical observations, and analogies between rodent behavior and human behavior.

• While the laboratory observations of the toxic effects of mercury are important, these observations do not explain how specific exposures in a rapidly developing infant affect certain tissues but not others where these mechanisms are also active.

• Laboratory studies also have not shown how these effects lead to autism.

Scientific Assessment:Biological Mechanisms Conclusion

In the absence of experimental or human evidence that either the MMR vaccine or vaccines containing thimerosal affect metabolic, developmental, immune, or other physiological or molecular mechanisms that are causally related to the development of autism, the committee concludes that the hypotheses generated to date are theoretical only.

Significance Assessment

The committee concludes that because autism can be such a devastating condition, any speculation that links vaccine and autism means that this is a significant issue.

Recommendations for Public Health Response

The committee recommends a public health response that fully supports an array of vaccine safety activities.

The committee recommends that available for funding for autism research be channeled to the most promising areas.

Recommendations for Public Health Response:

Policy Review

• No policy review of the licensure of the MMR vaccine or thimerosal-containing vaccines and or of current schedule and recommendations for administration of those vaccines.

Recommendations for Surveillance and Epidemiological Research

• Use standard and accepted case definitions and assessment protocols for ASD

• Conduct clinical and epidemiological studies of sufficient rigor to identify risk factors and biological markers of ASD

• Strengthen surveillance of adverse events – e.g., standardize case definitions of adverse events; establish

guidelines for use of VAERS; continue use of large linked databases and other tools; further develop of CISA.

Recommendations for Surveillance and Epidemiological Research (cont.)

• Conduct surveillance of ASD as exposure to thimerosal declines

• Increase efforts to quantify level of prenatal and postnatal exposure to thimerosal and other forms of mercury in infants, children, and pregnant women

Recommendations for Clinical Studies

• Because chelation therapy has potentially serious risks, the committee recommends that it be used only in carefully-controlled research settings with appropriate oversight by Institutional Review Boards protecting the interests of children who participate.

Recommendations for Communication

• Develop programs to increase public participation in vaccine safety research and policy decisions AND to enhance the skills and willingness of scientists and government officials to engage in constructive dialogue with the public about research findings and their implications for policy development.

Contact Information

• Phone: (202) 334-1342

• Email: imsafety@nas.edu

• Website: www.iom.edu/imsafety