Well, not entirely. With the first several seasons or three of covid, true enough it tended to mostly spare pediatric patients and was in fact statistically more likely to be severe in the elderly and/or those with other medical illnesses, but that was far from the whole story. During those rounds, for an individual presenting with covid pneumonia, meaning what killed during those rounds, other than statistics we had no way to predict who would progress to severe illness or death and who would slide on through. I had many young, healthy, 30-40+ year-olds, for example, who appeared not so bad initially but who did not survive and at the same time many elderly folks with extensive comorbidities who just marched on through.
Early on, for example, a 75-year-old with an LVAD and a nasty-looking chest Xray, did fine and with little help, same with a bilateral lung transplant 40-year-old. We also had quite a few folks in their 30s-40s+, young and healthy otherwise, who either did not survive or if they did only after a protracted hospital course.
The truth is, other than statistics, which do not apply to the individual except in a doesn't really help best-guess manner, we had no way to predict morbidity/mortality in the individual. That means susceptibility is not yet understood. I would guess a genetic factor not yet identified and/or viral load/timing, or something else entirely. In general with such things, it's often a combination of factors. Truth is we still know very little about this disease. But that's not even close to surprising, it is in fact pretty much normal for the process. What is surprising is how quickly we have progressed in that understanding. That mostly as a side effect of the now many years of efforts invested in viral research for HIV and hepatitis C. This rapid progression always has advantages and disadvantages. The disadvantage manifests as much of the evident confusion and chaos kicked around.....