Exponential epidemic growth has been clearly demonstrated for coronavirus disease 2019 (COVID-19) in every country it has touched, with ascertained cases growing from 25 at the start of March in Australia to over 6000 cases 6 weeks later. For every ascertained case, there may be anywhere up to nine infections that are not detected. 1 This silent component of spread is likely driven by asymptomatic 2 or mild infection, especially in younger people. In countries which restrict testing to symptomatic high risk people only, there will be silent growth of undetected infection until the epidemic is large enough to be felt in the health system. 3

The epidemic in China was largely localised through January 2020, with small numbers of imported cases in other countries. Following a lockdown on 23 January, it peaked on 5 February 4 and has declined since. However, as the cases decreased in China, COVID-19 began surging in other countries by late February. Asian countries such as South Korea took an aggressive approach to testing and achieved control. 5 In contrast, a more relaxed approach saw Europe becoming the new epicentre, followed by the United States, 6 which had epidemic growth because of major testing failures. In Italy, Spain and parts of the US, health systems capacity has been exceeded, with resulting shortages of intensive care beds and ventilators.