I looked at this question using the UK critical care data in a blog post at https://radfordneal.wordpress.com/2020/04/06/body-mass-and-risk-from-covid-19-and-in

I looked at this question using the UK critical care data in a blog post at https://radfordneal.wordpress.com/2020/04/06/body-mass-and-risk-from-covid-19-and-influenza/ https://radfordneal.wordpress.com/2020/04/06/body-mass-and-r... From that data, it does indeed seem that having BMI>40 is a large risk factor for COVID-19 being serious. BMI between 30 and 40 is also a risk factor, but much less than for BMI>40. In drawing these conclusions, I was assuming that being put in critical care was a good proxy for serious disease, but it's obviously not perfect - those who die quickly before reaching critical care obviously aren't counted, for instance. I think this is probably not a big issue for the UK, but reports are that quite substantial numbers of people in New York City died of COVID-19 without this being recognized by the health care system. Since we don't know (from this study) what the BMI was for these people it's hard to draw justified conclusions. It's frustrating that they don't break down the BMI<30 category further, since there's also reason to believe that low BMI could be a risk factor.