may wish to explore the assessment and characterization of these risks. Finally, it must be emphasized that the Immunization Safety Review Committee is not the panel being recommended. A new panel with specialized expertise related to communication issues is necessary.

SUMMARY

A substantial minority of parents (23–25%) participating in a recent survey agreed with the statement that getting too many vaccines is not good for a baby and can weaken the immune system (Gellin, 2000). But a review of the possible biological mechanisms for any adverse effects of multiple immunization on immune function suggests that the infant immune system is inherently capable of handling the numbers of antigens presented during routine immunization.

A review of the clinical and epidemiological literature favors rejection of a causal relationship between multiple immunizations and risk of infection and type 1 diabetes. The evidence was inadequate to accept or reject a causal relationship. Meanwhile, the biological evidence that immunization might lead to infection, autoimmune disease, or allergy is more than only theoretical. This literature base is somewhat limited, however.

Therefore, the committee recommends limited but continued public health attention to this issue in terms of capitalizing on current research efforts. No recommendations for policy review is made, but the committee does recommend an analysis of new frameworks for immunization policy, particularly as the number of licensed vaccines increases.



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Immunization Safety Review: Multiple Immunizations and Immune Dysfunction may wish to explore the assessment and characterization of these risks. Finally, it must be emphasized that the Immunization Safety Review Committee is not the panel being recommended. A new panel with specialized expertise related to communication issues is necessary. SUMMARY A substantial minority of parents (23–25%) participating in a recent survey agreed with the statement that getting too many vaccines is not good for a baby and can weaken the immune system (Gellin, 2000). But a review of the possible biological mechanisms for any adverse effects of multiple immunization on immune function suggests that the infant immune system is inherently capable of handling the numbers of antigens presented during routine immunization. A review of the clinical and epidemiological literature favors rejection of a causal relationship between multiple immunizations and risk of infection and type 1 diabetes. The evidence was inadequate to accept or reject a causal relationship. Meanwhile, the biological evidence that immunization might lead to infection, autoimmune disease, or allergy is more than only theoretical. This literature base is somewhat limited, however. Therefore, the committee recommends limited but continued public health attention to this issue in terms of capitalizing on current research efforts. No recommendations for policy review is made, but the committee does recommend an analysis of new frameworks for immunization policy, particularly as the number of licensed vaccines increases.

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Immunization Safety Review: Multiple Immunizations and Immune Dysfunction TABLE 6 Biological Mechanisms for the Possible Role of Immunizations in Increasing the Risk of Immune Dysfunction Adverse Health Outcome Mechanism Committee Conclusion About the Weight of the Biological Evidence Autoimmune disease Molecular mimicry Theoretical only   Bystander effect Weak Loss of protection induced by homologous infection Theoretical only Via the hygiene hypothesis Theoretical only Collective mechanistic possibilities Weak Allergic disease Bystander effect Weak   Via the hygiene hypothesis Theoretical only   Collective mechanistic possibilities Weak Heterologous Infections Carrier-induced epitope suppression Strong   Competition for antigen presentation  

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Immunization Safety Review: Multiple Immunizations and Immune Dysfunction BOX 1 Committee Conclusions and Recommendations SCIENTIFIC ASSESSMENT Causality Conclusions The committee concludes that the epidemiological and clinical evidence favors rejection of a causal relationship between multiple immunizations and an increased risk of heterologous infections. The committee concludes that the epidemiological and clinical evidence favors rejection of a causal relationship between multiple immunizations and an increased risk of type 1 diabetes. The committee concludes that the epidemiological and clinical evidence is inadequate to accept or reject a causal relationship between multiple immunizations and an increased risk of allergic disease, particularly asthma. Biological Mechanisms Conclusions Autoimmune Disease In the absence of experimental or human evidence regarding molecular mimicry or mercury-induced modification of any vaccine component to create an antigenic epitope capable of cross-reaction with self epitopes as a mechanism by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of autoimmunity, the committee concludes that these mechanisms are only theoretical. The committee concludes that there is weak evidence for bystander activation, alone or in concert with molecular mimicry, as a mechanism by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of autoimmunity. In the absence of experimental or human evidence regarding loss of protection against a homologous infection as a mechanism by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of autoimmunity, the committee concludes that this mechanism is only theoretical. In the absence of experimental or human evidence regarding mechanisms related to the hygiene hypothesis as a means by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of autoimmunity, the committee concludes that this mechanism is only theoretical. Considering molecular mimicry, bystander activation, and impaired immunoregulation collectively rather than individually, the committee concludes that there is weak evidence for these mechanisms as means by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of autoimmunity.

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Immunization Safety Review: Multiple Immunizations and Immune Dysfunction Allergic Disease The committee concludes that there is weak evidence for bystander activation as a mechanism by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of allergy. In the absence of experimental or human evidence regarding mechanisms related to the hygiene hypothesis as a means by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of allergy, the committee concludes that this mechanism is only theoretical. The committee concludes that there is weak evidence for the existence of any biological mechanisms, collectively or individually, by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk of allergy. Heterologous Infection The committee concludes that there is strong evidence for the existence of biological mechanisms by which multiple immunizations under the U.S. infant immunization schedule could possibly influence an individual’s risk for heterologous infections. SIGNIFICANCE ASSESSMENT Conclusions The committee concludes that concern about multiple immunizations has been, and could continue to be, of societal significance in terms of parental worries, potential health burdens, and future challenges for immunization policy-making. PUBLIC HEALTH RESPONSE RECOMMENDATIONS Policy Review The committee recommends that state and federal vaccine policymakers consider a broader and more explicit strategy for developing recommendations for the use of vaccines. The committee does not recommend a policy review—by the CDC’s Advisory Committee on Immunization Practices (ACIP), the American Academy of Pediatrics’ Committee on Infectious Diseases, and the American Academy of Family Physicians—of the current recommended childhood immunization schedule on the basis of concerns about immune system dysfunction. The committee does not recommend a policy review by the Food and Drug Administration’s Vaccines and Related Biologic Products Advisory

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Immunization Safety Review: Multiple Immunizations and Immune Dysfunction Committee of any currently licensed vaccines on the basis of concerns about immune system dysfunction. Research Epidemiological Research The committee recommends exploring the feasibility of using existing vaccine surveillance systems, alone or in combination, to study safety questions related to asthma and other important allergic disorders, as well as to study type 1 diabetes and other important autoimmune diseases. The committee recommends exploring the use of cohorts for research on possible vaccine-related disease risks. Furthermore, the committee recommends that disease registries and research programs for autoimmune and allergic disorders routinely collect immunization histories as part of their study protocol. Basic Science and Clinical Research The committee recommends continued research on the development of the human infant immune system. The committee endorses current research efforts aimed at identifying genetic variability in human immune system development and immune system responsiveness as a way to gain a better understanding of genetic susceptibility to vaccine-based adverse events. The committee recommends exploring the feasibility of collecting data on surrogate markers for type I diabetes and clinical history of allergic diseases in the vaccine testing and licensing process. The committee recommends exploring surrogates for type I diabetes and clinical history of allergic diseases in existing cohort studies of variations in the immunization schedule. Communication The committee recommends that an appropriate panel of multidisciplinary experts be convened by the Department of Health and Human Services. It would develop a comprehensive research strategy for knowledge leading to the optimal design and evaluation of vaccine risk-benefit communication approaches.