| Summary: | <p><strong>Context:</strong> A comprehensive understanding of the association between body mass index (BMI) and COVID-19 is still lacking.</p>
<p><strong>Objective:</strong> To investigate associations between BMI and risk of COVID-19 diagnosis, hospitalisation with COVID-19, and death after a COVID-19 diagnosis or hospitalisation (subsequent death), accounting for potential effect modification by age and sex.</p>
<p><strong>Design:</strong> Population-based cohort study.
Setting: Primary care records covering >80% of the Catalan population, linked to region-wide testing, hospital, and mortality records from March to May 2020.</p>
<p><strong>Participants:</strong> Adults (≥18 years) with at least one measurement of weight and height.</p>
<p><strong>Main outcome measures:</strong> Hazard ratios (HR) for each outcome.</p>
<p><strong>Results:</strong> We included 2 524 926 participants. After 67 days of follow-up, 57 443 individuals were diagnosed with COVID-19, 10 862 were hospitalised with COVID-19, and 2467 had a subsequent death. BMI was positively associated with being diagnosed and 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 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 association between BMI and subsequent death was J-shaped, with a modestly higher risk of death among individuals with BMIs ≤19kg/m2 and a more pronounced increasing risk for BMIs ≥40kg/m2. The increase in risk for COVID-19 outcomes was particularly pronounced among younger patients.</p>
<p><strong>Conclusions:</strong> There is a monotonic association between BMI and COVID-19 diagnosis and hospitalisation risks, but a J-shaped one with mortality. More research is needed to unravel the mechanisms underlying these relationships.</p>
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