The self-controlled case series (SCCS) design is widely used in studies of rare events, for example of vaccine side effects. Conceptually, the design has two main features. First, it requires only data from individuals who have experienced the event of interest. Therefore, the design is especially useful when the outcome is rare and the available data come from surveillance or clinical records, say, of hospitalized patients. Second, because comparisons are made within single individuals, time-invariant confounding between treatment and outcome is eliminated. We formulate a set of sufficient identification assumptions for estimating the causal effect of vaccination on an adverse outcome under the SCCS design, drawing on ideas from the causal inference literature. Unlike the existing SCCS literature, we do not consider a parametric model as the starting point. This allows us to establish when and why the SCCS design allows identification of causal effects, using data from cases only. However, because the sufficient identifiability assumptions are implausible in some applications, we also develop statistical tests for the existence of a treatment effect that remain valid when only a subset of the identification assumptions holds.

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