How many people in Britain have already been infected with the coronavirus? The truth is, we don’t really know. Yet, this is a very important number because, without it, it is difficult to properly assess the severity of the virus, its likely trajectory, and the consequent burden on health and social care systems. Using a random sample survey and self-reported data on infection and symptoms, Patrick Sturgis and Jouni Kuha (LSE) find a symptomatic infection rate in the general population of Great Britain of approximately 10 per cent, equating to around 5 million people, by the end of April