The reproduction measure hides a wealth of complications but is indispensable to decision-makingby Philip Ball / May 21, 2020 / Leave a comment
Once you get past the astonishing mathematical illiteracy with which the UK government has tried to convey its Covid-19 “alert level” in preparation for an eventual exit from lockdown, it does make some sense. The two most relevant factors for the spread of the disease are indeed the level of infections and the reproduction number R, the latter being a measure of how readily the disease is being transmitted between people. (The government’s five criteria for relaxing the restrictions also take into account the availability of testing and protective equipment and the capacity of the health services.)
Suddenly everyone is talking about R. Is it rising or falling? Is it higher in cities, or in the north of England? What does it really mean?
The first thing to understand is that R does not quantify some fundamental feature of nature, like the strength of the Earth’s gravity or the speed of light in a vacuum. It emerges from specific epidemiological models, and depends on the context in which the virus is spreading. It will vary not just from country to country but from town to town, and is contingent on how individuals are acting in particular, how well they are socially distancing to reduce infection.
In effect R—the reproduction number—denotes the average number of people an infected individual passes the disease on to. If this value is less than 1, the disease cannot keep spreading, and an epidemic will die out. The larger it is, the more contagious the condition and the faster it spreads. The basic rate at which the virus is transmitted in a population in normal (pre-lockdown) circumstances, denoted R0 (“R zero”—not to be confused with an R value of zero!), is a rough-and-ready measure of how intrinsically infectious the virus is—although it depends also on other factors such as the size of the population and how many people in it are susceptible in the first place. Strictly speaking, the national and regional R values now being estimated are “effective reproduction numbers,” more properly denoted Re. These can and usually will vary over time and place, depending on—among other things—how people are behaving and what safety measures are in place.
At the outset of the Covid-19 pandemic there was a rush to pin down the virus’s R0 so as to better understand what…