ABSTRACT
Objective: To investigate associations between body mass index (BMI) and risk of COVID-19 diagnosis, hospitalisation with COVID-19, and COVID-19-related death, accounting for potential effect modification by age and sex. Design: Population-based cohort study. Setting: Primary care records covering >80% of the Catalonian population (Spain), linked to regionwide testing, hospital, and mortality records from March to May 2020. Participants: People aged [≥]18 years with at least one measurement of weight and height from the general population and with at least one year of prior medical history available. Main outcome measures: Cause-specific hazard ratios (HR) with 95% confidence intervals for each outcome. Results: Overall, 2,524,926 participants were followed up for a median of 67 days. A total of 57,443 individuals were diagnosed with COVID-19, 10,862 were hospitalised with COVID-19, and 2,467 had a COVID-19-related death. BMI was positively associated with being diagnosed as well as hospitalised with COVID-19. Compared to a BMI of 22kg/m2, the HR (95%CI) of a BMI of 31kg/m2 was 1.22 (1.19-1.24) for COVID-19 diagnosis, and 1.88 (1.75-2.03) and 2.01 (1.86-2.18) for hospitalisation without and with a prior outpatient diagnosis, respectively. The relation between BMI and risk of COVID-19 related death was J-shaped. There was a modestly higher risk of death among individuals with BMIs[≤]19 kg/m2 and a more pronounced increasing risk for BMIs [≥]37 kg/m2 and [≥]40kg/m2 among those who were previously hospitalised with COVID-19 and diagnosed with COVID-19 in outpatient settings, respectively. The increase in risk for COVID-19 outcomes was particularly pronounced among younger patients. Conclusions: There is a monotonic association between BMI and COVID-19 infection and hospitalisation risks, but a J-shaped one with mortality. More research is needed to unravel the mechanisms underlying these relationships.