Knowing the size of an epidemic and whether it is increasing or decreasing is core to any response to it, be it by individuals, organisations, or governments. Julii Brainard and colleagues1 have compared 12 COVID-19 surveillance systems that were used in England from the start of the second wave of the pandemic (Sept 1, 2020) to just before Omicron emerged (Nov 30, 2021). Compared with the most accurate measures from the Office for National Statistics (those most representative of the whole population), which are least timely (10–24-day lag), they found that “laboratory-confirmed case counts and emergency department attendances were the most timely and also independent indicators of concurrent epidemic status”.
