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

It is clear from the data that has emerged from China that there is a significant increase in the CFR with age. Our results suggest a very low fatality ratio in those under the age of 20. However, as there are very few cases in this age-group, it remains unclear whether this reflects a low risk of death or a difference in susceptibility. Serological testing in this age-group will therefore be critical in the coming weeks to understand the significance of this age-group in driving population transmission. There is a clear increase in the estimated CFR from the age of 50 upwards, with this proportion rising from approximately 1% in the 50-59 age-group to 13% in those aged 80 and above. This increase in severity with age is clearly reflected in case-reports in which the mean age tends to be in the range 50-60 years. Different surveillance systems will pick up a different age-case-mix and we find that those with milder symptoms detected through history of travel are younger on average than those detected through hospital surveillance. Our correction for this surveillance bias therefore allows us to obtain estimates that can be applied to different case-mixes and demographic population structures. However, it should be noted that this correction is applicable under the assumption of a uniform infection attack rate (i.e. exposure) across the population.