How many are at increased risk of severe COVID-19 disease? Rapid global, regional and national estimates for 2020

Andrew Clark; Mark Jit; Charlotte Warren-Gash; Bruce Guthrie; Harry HX Wang; Stewart W Mercer; Colin Sanderson; Martin McKee; Christopher Troeger; Kanyin I Ong; Francesco Checchi; Pablo Perel; Sarah Joseph; Hamish P Gibbs; Amitava Banerjee; Rosalind M Eggo

The GBD study provides prevalence estimates for each disease category separately, but not for the prevalence of people in at least 1 of these categories. One problem was that diseases may cluster, for example if they are causally related, and we incorporated this effect for each country, sex and age group (but we omit subscripts for clarity). We first calculated , which is the expected proportion of individuals with at least one condition assuming that the prevalences of the underlying conditions are independent (e.g. the fact that someone has diabetes does not affect their risk of getting cancer) as 1 minus the probability of not having any of the conditions c1, c2, c3….i.e. 1 -(1 -p_c1) x (1 -p_c2) x (1 -p_c3)…. We then estimated the proportion , who have at least one underlying condition as = × , where is the ratio between the observed and expected percentage of individuals with at least one condition. The ratio is drawn from evidence from two large cross-sectional multimorbidity studies in Scotland 17 and Southern China 18 (supplementary appendix).