World leaders and public health experts are poised to spend the coming months or years obsessed with a variable known as R0.
Pronounced “R-naught,” it represents the number of new infections estimated to stem from a single case.
In other words, if R0 is 2.5, then one person with the disease is expected to infect, on average, 2.5 others.
An R0 below 1 suggests that the number of cases is shrinking, possibly allowing societies to open back up. An R0 above 1 indicates that the number of cases is growing, perhaps necessitating renewed lockdowns or other measures.
But R0 is messier than it might look. It is built on hard science, forensic investigation, complex mathematical models — and often a good deal of guesswork. It can vary radically from place to place and day to day, pushed up or down by local conditions and human behavior.
Yet for all its vagaries, R0 is expected to shape our world in the coming months and possibly years as governments and health experts treat it as the closest thing to a compass in navigating the pandemic.
What follows is a simple guide to how this metric works, why it matters and how to think about it.