Commentary on the use of the reproduction number R during the COVID-19 pandemic.

ABSTRACT

Since the beginning of the COVID-19 pandemic, the reproduction number [Formula see text] has become a popular epidemiological metric used to communicate the state of the epidemic. At its most basic, [Formula see text] is defined as the average number of secondary infections caused by one primary infected individual. [Formula see text] seems convenient, because the epidemic is expanding if [Formula see text] and contracting if [Formula see text]. The magnitude of [Formula see text] indicates by how much transmission needs to be reduced to control the epidemic. Using [Formula see text] in a naïve way can cause new problems. The reasons for this are threefold (1) There is not just one definition of [Formula see text] but many, and the precise definition of [Formula see text] affects both its estimated value and how it should be interpreted. (2) Even with a particular clearly defined [Formula see text], there may be different statistical methods used to estimate its value, and the choice of method will affect the estimate. (3) The availability and type of data used to estimate [Formula see text] vary, and it is not always clear what data should be included in the estimation. In this review, we discuss when [Formula see text] is useful, when it may be of use but needs to be interpreted with care, and when it may be an inappropriate indicator of the progress of the epidemic. We also argue that careful definition of [Formula see text], and the data and methods used to estimate it, can make [Formula see text] a more useful metric for future management of the epidemic.