Estimates of the severity of COVID-19 disease

Robert Verity; Lucy C Okell; Ilaria Dorigatti; Peter Winskill; Charles Whittaker; Natsuko Imai; Gina Cuomo-Dannenburg; Hayley Thompson; Patrick Walker; Han Fu; Amy Dighe; Jamie Griffin; Anne Cori; Marc Baguelin; Sangeeta Bhatia; Adhiratha Boonyasiri; Zulma M Cucunuba; Rich Fitzjohn; Katy A M Gaythorpe; Will Green; Arran Hamlet; Wes Hinsley; Daniel Laydon; Gemma Nedjati-Gilani; Steven Riley; Sabine van-Elsand; Erik Volz; Haowei Wang; Yuanrong Wang; Xiayoue Xi; Christl Donnelly; Azra Ghani; Neil Ferguson

Assessing the severity of COVID-19 is critical to determine both the appropriateness of mitigation strategies and to enable planning for healthcare needs as epidemics unfold. However, crude case fatality ratios (CFRs) obtained from dividing deaths by cases can be misleading 9,10 . Firstly, there can be a period of two to three weeks between a case developing symptoms, subsequently being detected and reported and observing the final clinical outcome. During a growing epidemic the final clinical outcome of most of the reported cases is typically unknown. Simply dividing the cumulative reported number of deaths by the cumulative number of reported cases will therefore underestimate the true CFR early in an epidemic 9-11 . This effect was observed in past epidemics of respiratory pathogensincluding SARS 12 and H1N1 9 influenza -and as such is widely recognised. Thus, many of the estimates of the CFR that have been obtained to date for COVID-19 correct for this effect [13] [14] [15] [16] . Additionally, however, during the exponentially growing phase of an epidemic, the observed time-lags between the onset of symptoms and outcome (recovery or death) are censored and naïve estimates of the observed times from symptoms onset to outcome provide biased estimates of the actual distributions. Ignoring this effect tends to bias the estimated CFR downwards during the early growth phase of an epidemic.