What the reproductive number [Formula: see text] can and cannot tell us about COVID-19 dynamics

Shaw, Clara L.; Kennedy, David A.

The reproductive number [Formula: see text] (or [Formula: see text] , the initial reproductive number in an immune-naïve population) has long been successfully used to predict the likelihood of pathogen invasion, to gauge the potential severity of an epidemic, and to set policy around interventions. However, often ignored complexities have generated confusion around use of the metric. This is particularly apparent with the emergent pandemic virus SARS-CoV-2, the causative agent of COVID-19. We address some misconceptions about the predictive ability of the reproductive number, focusing on how it changes over time, varies over space, and relates to epidemic size by referencing the mathematical definition of [Formula: see text] and examples from the current pandemic. We hope that a better appreciation of the uses, nuances, and limitations of [Formula: see text] and [Formula: see text] facilitates a better understanding of epidemic spread, epidemic severity, and the effects of interventions in the context of SARS-CoV-2.