A century has passed since the great 1918 pandemic, in which tens of millions died. When an anniversary of a major event comes round, we can ask what if the event were to occur today. Catastrophe modelers can also reimagine the event being different from what it actually was. This counterfactual perspective leads to important insights into pandemic risk, which have only recently emerged from recent virological research.

This paper includes the latest understandings of how previous exposure to influenza strains affects antibody response, the principle of antigenic imprinting.