Severe Combined Immunodeficient (SCID) Mice in Vaccine Assessment

New or improved vaccines require testing before they are licensed for use in the general population. Although toxicity and immunological data can be obtained from Phase I human trials, in the vast majority of cases it is not possible to demonstrate efficacy by challenging vaccine recipients with the specific infectious agent. Therefore, prior to large-scale production of vaccines and their assessment in Phase II and III clinical trials, there is a requirement for in vivo studies that can predict the level of protection that may be expected from a vaccine. Although many animal models are available for infectious disease studies, they all rely on the animal’s own immune system for the generation of the protective responses, and these may not reflect the immunological responses generated in humans. The route of challenge that must be used in order to achieve infection is often not the same as the one that occurs in human disease, and this further confounds the relevance of the animal model. In addition, some infections are specific to humans, and it is sometimes impossible to find a suitable animal species that human pathogens will infect. This may be because of the lack of appropriate receptors, the presence of a functional immune system able to eliminate the pathogen, or the fact that models cannot always be relied on to re-create the normal pathophysiology. In some cases, this resistance to infection can be overcome by the development of a transgenic animal in which the appropriate receptors are expressed. The development of transgenic models has been directed toward genetic disorders or dissection of the immune system by gene-knockout technology.