Cast your mind back to the height of the Covid-19 pandemic, when the world seemed to have a single shared obsession - the “R number”. This simple metric, which measured the reproductive rate of the virus, gave a tangible sense of the invisible threat that lurked around us. Was it above one, meaning that the virus was spreading exponentially? Or had lockdowns and other public health measures successfully brought it below that threshold? The R number became a predictor of the public mood li...
