Be careful because R0 and Rt refer to different things: R0 is a property of the pathogen under the assumption of “normal life” and measures how inherently infec

Be careful because R0 and Rt refer to different things: R0 is a property of the pathogen under the assumption of “normal life” and measures how inherently infectious it is. Rt is a measure of the real-life infectivity of a pathogen at a specific point in time, under no assumption of “normal life”, and possibly within some arbitrary geographical bounds. The purpose of Rt is not to measure the inherent infectivity of a pathogen, but rather its infectivity in a specific environment and set of circumstances: it’s the high-level metric that informs you of the combined impact of public health measures, social behaviour changes, immunity, etc, on the pathogen’s infectivity. You can see Rt as being equal to R0 times some correction factor that accounts for everything else.