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

Emerging evidence from China, Europe and the USA has shown a consistently higher risk of severe COVID-19 disease in older individuals and those with underlying health conditions. 1-3 Mild and moderate cases are those with symptoms but without evidence of pneumonia or dyspnea, and low mortality is expected in this group. Severe cases have those symptoms, would require hospitalisation if available, and are at risk of increased mortality. 4, 5 In a recent report from the USA, underlying conditions were reported in 71% (732/1037) of individuals hospitalised with COVID-19 and 94% (173/184) of deaths. 1 The World Health Organization (WHO) as well as public health agencies in countries including the UK and US, have issued guidelines on who is considered to be at increased risk of severe COVID-19 illness. [6] [7] [8] This includes individuals with cardiovascular disease (CVD), chronic kidney disease (CKD), diabetes, chronic respiratory disease (CRD) and a range of other chronic conditions. Such conditions increase the risk of needing hospital-based treatment such as oxygen supplementation or mechanical ventilation. A large proportion of the additional health care burden of COVID-19 epidemics is likely to result from infection of those with underlying conditions. Identifying at-risk populations is important not only for making projections of the likely health burden in countries, 9, 10 but also for the design of effective strategies that aim to reduce the risk of transmission to people in target groups. This is sometimes termed shielding, defined as "a measure to protect extremely vulnerable people by minimising interaction between those who are extremely vulnerable and others." 11 It has the potential to reduce mortality in vulnerable groups (direct benefits), while at the same time mitigating the expected surge in demand for hospital beds (indirect benefits), which is critical where systems risk being strained by the national epidemic. However, trying to shield an excessive proportion of a population may strain country resources and reduce the overall effectiveness of shielding. A detailed assessment of the number of at-risk individuals can inform possible shielding strategies. If a vaccine becomes available in the future, it could also be used to inform the process of prioritising different groups, based on risk.