<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>This study aimed to describe the population at risk of severe COVID-19 due to underlying health conditions across the United Kingdom in 2019.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We used anonymised electronic health records from the Clinical Practice Research Datalink GOLD to describe the point prevalence on 5 March 2019 of the at-risk population following national guidance. Prevalence for any risk condition and for each individual condition is given overall and stratified by age and region. We repeated the analysis on 5 March 2014 for full regional representation and to describe prevalence of underlying health conditions in pregnancy. We additionally described the population of cancer survivors, and assessed the value of linked secondary care records for ascertaining COVID-19 at-risk status.</jats:p></jats:sec><jats:sec><jats:title>Findings</jats:title><jats:p>On 5 March 2019, 24·4% of the UK population were at risk due to a record of at least one underlying health condition, including 8·3% of school-aged children, 19·6% of working-aged adults, and 66·2% of individuals aged 70 years or more. 7·1% of the population had multimorbidity. The size of the at-risk population was stable over time comparing 2014 to 2019, despite increases in chronic liver disease and diabetes and decreases in chronic kidney disease and current asthma. Separately, 1·6% of the population had a new diagnosis of cancer in the past five years.</jats:p></jats:sec><jats:sec><jats:title>Interpretation</jats:title><jats:p>The population at risk of severe COVID-19 (aged ≥70 years, or with an underlying health condition) comprises 18.5 million individuals in the UK, including a considerable proportion of school-aged and working-aged individuals.</jats:p></jats:sec><jats:sec><jats:title>Funding</jats:title><jats:p>NIHR HPRU in Immunisation</jats:p></jats:sec><jats:sec><jats:title>Research in context</jats:title><jats:sec><jats:title>Evidence before this study</jats:title><jats:p>We searched Pubmed for peer-reviewed articles, preprints, and research reports on the size and distribution of the population at risk of severe COVID. We used the terms (1) risk factor or co-morbidity or similar (2) COVID or SARS or similar and (3) prevalence to search for studies aiming to quantify the COVID-19 at-risk UK population published in the previous year to 19 July 2020, with no language restrictions. We found one study which modelled prevalence of risk factors based on the Global Burden of Disease (which included the UK) and one study which estimated that 8.4 million individuals aged ≥30 years in the UK were at risk based on prevalence of a subset of relevant conditions in England. There were no studies which described the complete COVID-19 at-risk population across the UK.</jats:p></jats:sec><jats:sec><jats:title>Added value of this study</jats:title><jats:p>We used a large, nationally-representative dataset based on electronic health records to estimate prevalence of increased risk of severe COVID-19 across the United Kingdom, including all conditions in national guidance. We stratified by age, sex and region to enable regionally-tailored prediction of COVID-19-related healthcare burden and interventions to reduce transmission of infection, and planning and modelling of vaccination of the at-risk population. We also quantified the value of linked secondary care records to supplement primary care records.</jats:p></jats:sec><jats:sec><jats:title>Implications of all the available evidence</jats:title><jats:p>Individuals at moderate or high risk of severe COVID-19 according to current national guidance (aged ≥70 years, or with a specified underlying health condition) comprise 18·5 million individuals in the United Kingdom, rather than the 8.43 million previously estimated.</jats:p><jats:p>The 8·3% of school-aged children and 19·6% of working-aged adults considered at-risk according to national guidance emphasises the need to consider younger at-risk individuals in shielding policies and when re-opening schools and workplaces, but also supports prioritising vaccination based on age and condition-specific mortality risk, rather than targeting all individuals with underlying conditions, who form a large population even among younger age groups.</jats:p><jats:p>Among individuals aged ≥70 years, 66·2% had at least one underlying health condition, suggesting an age-targeted approach to vaccination may efficiently target individuals at risk of severe COVID-19.</jats:p><jats:p>These national estimates broadly support the use of Global Burden of Disease modelled estimates and age-targeted vaccination strategies in other countries.</jats:p></jats:sec></jats:sec>