Interventions for Tuberculosis Control and Elimination
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An alternative, much less expensive approach to assess the protective effect of BCG is the use of a retrospective study design. The best known example is the case-control study. Here one starts with the outcome and looks at the exposure.
Cases of tuberculosis (the outcome) are assembled and a group that is comparable in every respect but the outcome (they must be free of disease) is chosen as a control group. The prevalence of BCG vaccination is then assessed and compared in the two groups. Rates cannot be calculated as this type of study is not population-based. The measure here is the odds ratio. It approximates the relative risk for rare diseases. The protective effectiveness is similarly calculated as 1 minus the odds ratio, usually expressed as a percentage.
Case-control studies are more likely to be biased than prospective studies as the selection of appropriate controls is often hampered with difficulties that cannot always be easily overcome.
