Rt: the effective reproduction number
It’s Easter. A couple weeks ago, this was the target for returning to normal. Hospitals are now full of patients, cities bloom as new hotspots, and politicians wrestle with the balance of human and economic costs. We’re left to wonder if we are equipped with the right metrics to guide our path forward. Add to the confusion that metrics are based on noisy data that changes daily. There’s one metric, however, that has the most promise. It’s called Rt – the effective reproduction number. We can estimate it, and it’s the key to getting us through the next few months.
Most people are more familiar with R0. R0 is the basic reproduction number of an epidemic. It’s defined as the number of secondary infections produced by a single infection. If R0 is greater than one, the epidemic spreads through every susceptible individual in a population. If R0 is less than one, the epidemic spreads, but limps along and disappears before everyone becomes infected. The flu has an R0 between one and two while measles sits in the high teens. While R0 is a useful measure, it is flawed in an important way: it’s static.
We’ve all witnessed that humans are adaptable. Our behavior changes, whether mandated or self-prescribed, and that changes the effective R value at any point in time. As we socially distance and isolate, R plummets. Because the value changes so rapidly, Epidemiologists have argued that the only true way to combat COVID19 is to understand and manage by Rt.