Case-case study of a Salmonella outbreak: an epidemiologic method to analyse surveillance data.

Routine infectious disease surveillance data has to be analysed quickly in order to prevent further disease transmission. This can be done by using epidemiological studies, such as case-control studies. However, these studies are complex to perform and susceptible for biases. An alternative is the case-case study design which is less prone to such limitations. So far mainly methodological publications of this study design exist. In this investigation, outbreak cases are compared with cases infected with the same disease. A Salmonella outbreak was analysed with a case-case study design in order to test the applicability of this method. The analysis showed significant associations to the consumption beef (OR 11.8; 95% CI 2.4-66.7; p<0.001) and pork (OR 8.44; 95% CI 1.7-46.4; p<0.001). A case-control study on the same outbreak confirmed these results. An infected control group is very comparable to the outbreak cluster, which limits selection and recall bias. However, the calculated OR has to be interpreted in context with the study design. The case-case study design has proven to be a useful tool to analyse surveillance data. It was easy to perform and had methodological benefits.