Case-case comparisons to study causation of common infectious diseases

Background Methods Results Conclusions Analytical studies of reportable inf ectious diseases often use the small minority of cases detected through sur veillance systems. For many diseases, notification of cases represents a no n-random selection process. Apparent differences in exposure histories may be due to biases involved in the surveillance system selection of cases com pared to randomly selected controls. In addition, differential recall betwe en cases and controls may occur. One way to avoid these problems is to comp are cases with another group of cases with a different disorder selected by a similar surveillance system, although this can introduce new biases. Methods In infectious diseases cases with the same disease can be divided i nto aetiologically meaningful subgroups by subtyping the pathogen. Exposure history can then be compared between these subgroups. Results Several biases are removed. The control group composed of other cas es does not represent the exposure history of the study base but differs fr om it in a predictable and useful way. People considered as controls will h ave a higher incidence of general predisposing factors than the general pop ulation. Analysis is limited to factors associated with exposure to the inf ecting agent. Conclusion Case-case comparison is a development of case-control methodolog y made possible by laboratory typing techniques. These comparisons allow a more restricted but more refined analysis of the association of some exposu res with infection. Determination of how exposure to the infectious agent o ccurred is more efficient and unbiased than in standard case-control studie s but general factors determining whether disease occurs after an infectiou s exposure can not be studied.