A comprehensive understanding of the association between body mass index (BMI) and COVID-19 is still lacking.

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.

Population-based cohort study.

Primary care records covering >80% of the Catalan population, linked to region-wide testing, hospital, and mortality records from March to May 2020.

Adults (≥18 years) with at least one measurement of weight and height.

Hazard ratios (HR) for each outcome.

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/m ², the HR (95%CI) of a BMI of 31kg/m ² 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/m ² and a more pronounced increasing risk for BMIs ≥40kg/m ². The increase in risk for COVID-19 outcomes was particularly pronounced among younger patients.

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.