Models not only represent but may also influence their targets in important ways. While models' abilities to influence outcomes has been studied in the context of economic models, often under the label ‘per-formativity', we argue that this phenomenon also pertains to epidemiological models, such as those used for forecasting the trajectory of the Covid-19 pandemic. After identifying three ways in which a model by the Covid-19 Response Team at Imperial College London (Ferguson et al. 2020) may have influenced scientific advice, policy, and individual responses, we consider the implications of epidemiological models' performative capacities. We argue, first, that performativity may impair models' ability to successfully predict the course of an epidemic; but second, that it may provide an additional sense in which these models can be successful, namely by changing the course of an epidemic.