between immunization and a particular adverse event may be found to be biologically plausible at the same time that the epidemiological evidence is found to be inadequate to accept or reject a causal relationship.

Given the resulting lack of clarity, the committee believes that the adoption of new terminology and a new approach to its discussions of experimental biological data are warranted. The committee will thus review evidence regarding “biological mechanisms” that might be consistent with the proposed relationship between immunization and a given adverse event. This biological assessment section of the report is written distinct from any argument regarding the causality of such relationships.

Beginning with this report, the committee will summarize the biological mechanisms as theoretical only, or as having derived from either experimental evidence in animals or in vitro systems or from mechanism-related, biological evidence in humans of response to vaccine or infectious disease antigen. If there is either experimental evidence (e.g., from animals) or evidence in humans for a mechanism, the committee will designate it as weak, moderate, or strong. Though the committee tends to judge biological evidence in humans to be “stronger” than experimental evidence, the strength of the evidence also depends on other factors, such as experimental design and sample size. The conclusions drawn from this review will depend both on evidence and scientific judgment.


Over the past two decades, the pediatric immunization schedule has grown more complicated. In 1980, infants received immunizations against four diseases (diphtheria, tetanus, pertussis, and polio). Today, a healthy infant immunized in complete accord with the recommended childhood immunization schedule receives up to 15 doses of five vaccines to protect against seven diseases by 6 months of age and up to 20 doses of seven vaccines to protect against 11 diseases by 2 years of age. According to a recent survey, a substantial minority of parents (23–25%) believes that getting too many immunizations weakens a child’s immune system and that children get more immunizations than are good for them (Gellin et al., 2000).

The Immunization Safety Review Committee was asked to address the hypothesis that multiple immunizations can adversely affect the developing immune system. One particular concern, for example, is related to increases in the incidence of diseases such as asthma and type 1 diabetes—conditions associated with immune system dysfunctions. Although genetic factors are known to affect the risk of these diseases, increases in their incidence seem more likely to reflect changes in environmental exposures than in the genetic makeup of a population. Immunization has been proposed as one possible adverse environmental modifier of immune function.

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