R0 is the average expected number of secondary infections for each primary case in a naive and wholly susceptible population with no prior history of the disease. The R0 captures the capacity of a pathogen to start an outbreak, and it reflects the degree to which it is infectious in the absence of any measures to control it. Re, however, reflects the real-time spread of the epidemic later in its course, when the population is no longer “naive.” The Re is susceptible to human responses.
