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The simplistic conventional model of immunisation is no longer valid: we cannot assume that a vaccine acts independently from other vaccines or that it influences only infections caused by its target disease. For example, there is now evidence that measles vaccine reduces mortality from infections other than measles and that Bacillus Calmette–Guerin vaccine reduces mortality from infections other than tuberculosis.
How you respond to a vaccine is influenced by the infections and immunisations you have had in the past, and the vaccine you are given today will influence your future response to immunisation with other vaccines and your future response to infections — including infections with unrelated (heterologous) organisms.1,–,6 Heterologous immunity has been studied by research groups based at the Australian National University,1 the University of Massachusetts Medical School2 4 5 and Imperial College London.6 Some of the mechanisms are now understood, and the phenomenon is widely recognised by immunologists. However, the importance of infection history is still overlooked in most medical models of infectious disease and in most vaccination studies.6 7
Unfortunately, the nomenclature is confused. Immunologists call an immune response heterologous when exposure to one organism alters the response to an unrelated organism and cross-reactive when the organisms are non-identical but closely related; non-specific or innate immunity refers to defence mechanisms that do not need prior exposure to the organism. However, epidemiologists often say that a vaccine has non-specific effects when it alters the immune response to unrelated (heterologous) organisms. To further complicate matters, “cross reactions” and “immunity, innate” are medical subject headings, but heterologous immunity is covered only by general terms such as “immunologic memory” and “T lymphocytes”.
It is likely that more than 3 million deaths from measles, tetanus and pertussis are prevented by immunisation each year. …
Competing interests None.
Provenance and peer review Commissioned; externally peer reviewed.
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