Body Mass and Risk from COVID-19 and Influenza

formalized as Body Mass Index (BMI, weight in kilograms divided by squared height in metres)

Seriously, BMI is a terrible metric. It is hardly any better than just plan weight. In Ancel Keys' original paper he was looking for a way to estimate subcutaneous fat. Two problems here a) subcutaneous fat is pretty benign - visceral fat is the problem, b) BMI badly estimates even subcutaneous fat. So it measures the wrong thing, badly.

In many studies BMI in the slightly overweight range has the lowest mortality. BMI ignores body composition. Thus people who are skinny-fat and metabolically healthy show up as a 'healthy' BMI. Athletes, fit muscular people show up as overweight or obese. BMI does not work for smokers and ex smokers, older people, the skinny fat, athletes and muscular people - in general it works for perhaps slightly less than half the population.

The continued use of BMI is a good example of the problem in medicine of incumbency bias - beliefs and practice that are not based on solid evidence persist and can only be displaced by overwhelming evidence, and maybe not even then.

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0039504 - in particular have a look at table 2.

Note in the first study referred to in OP's link the excess risk from being overweight is not statistically significant and even the excess risk from being obese is small. But even a bad metric like BMI is enough to see that being morbidly obese is bad. The risk from being underweight is often higher and may be attributed (as OP mentioned) to current or past smoking, but also to prior or current illness.

In both studies we see the typical U shaped curve for BMI and bad things. ABSI typically has a monotonic curve and is robust to issues like smoking. It is a real pity these studies did not use a decent metric.

In the CCP Corona Virus study we see that people who are "overweight" are *less* likely to be in critical care than the general public.

There are various hypotheses about why morbidly obese people are more vulnerable to CCP Corona virus. Perhaps it is related to metabolic syndrome as high or high normal serum glucose tends to depress the immune system.