  The reproductive number R_0 (and its value after initial disease emergence R)
has long been 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 of these
misconceptions, namely, how R changes over time, varies over space, and relates
to epidemic size by referencing the mathematical definition of R and examples
from the current pandemic. We hope that a better appreciation of the uses,
nuances, and limitations of R facilitates a better understanding of epidemic
spread, epidemic severity, and the effects of interventions in the context of
SARS-CoV-2.
