To the committee’s knowledge, realistic animal models that could provide information on the potential of long-term health outcomes of the full immunization schedule in humans are not available. Furthermore, an assessment of the long-term effects of multiple immunizations in, for example, rats 3 months after they receive those immunizations would not be applicable to humans because the onset of such chronic diseases takes years to arise in humans.
An example of an animal model is the model of allergic hypersensitivity to dust mites and Ascaris in monkeys, which has resulted in studies of asthma (Hogan et al., 1994). However, few such primate colonies with relevance to asthma in humans exist. Furthermore, the cost to establish and maintain a primate colony is extremely high, and the availability of allergic monkeys is therefore extremely limited.
In the absence of animal models of the spontaneous onset of chronic diseases such as Guillain-Barré syndrome, studies of the effects of multiple vaccinations on aspects of airway hyperreactivity in mice or monkeys could be performed, but such studies would be limited in their ability to answer questions about the aggregate immunization schedule.
The key limitation to the use of animal models for evaluation of the immunization schedule therefore is not the availability of science or resources but the limited ability of models to produce results generalizable to the human experience. Given the committee’s recognition of the complexity of the immunization schedule, the importance of family history, the role of individual immunologic factors, and the complex interaction of immunization with the health care system, the committee determined that it would be more appropriate to focus future research efforts on human research rather than research involving animal models.
In summary, it is not possible to recommend studies with animals to inform the notion that the aggregate childhood immunization schedule results in the onset of chronic diseases. The committee also recognized the role of animal models in understanding neurological diseases, which have made important contributions to the understanding of disease processes that affect the brain in terms of structural or motor changes, such as seizures. In addition to the limitations described above in relation to chronic diseases, the study of neurological diseases such as autism has limited use for animal models since “no animal embodies the repertoire of behaviors seen in the human, and in particular, no animal has language equivalent to that of the human” (IOM, 2012, p. 86). Thus, there are sizable barriers in using animal models to assess such neurological outcomes following administration of the childhood immunization schedule.