My daughter almost died from a far more common virus (EBV) age 6.
18 months post diagnosis we are still waiting for her genome sequencing to complete. It’s likely she has a gene fault somewhere, no one knows where it is though. Having a gene fault doesn’t mean you will automatically develop a disease though, just that you are more likely to than people without it (the BRCA faults are probably the best known examples of this).
In my daughter’s case, EBV set off a Cytokine Storm, she was ventilated for 5 days. The same reaction is what’s happening in COVID19 patients.
So yes, many people who die from this will probably have something underlying in their genetic code, but it might not express in any other way. My daughter took a year to recover, but she was never ill before of since beyond an ear infection or a cold.
Viruses do weird things. 95% of the world have had EBV. Most won’t even notice and will just assume it’s a cold. Young teens may develop long term fatigue through glandular fever. In some people it’s not a problem at the time but leads to EBV related cancers later on. Some, like my daughter, will have a reaction that is 100% fatal without treatment.
No one knows much about the different ways SARS-CoV-2 will express long term. Children don’t seem to be getting particularly sick (not even immunocompromised ones). One theory is that human growth hormone is acting in a protective way, which is why teenagers are slightly more susceptible than younger kids. Once you’ve had your final pubertal growth spurt the hormone tails off. It’s just theory until a study proves it though.
It’s all so early, we just don’t know much about this virus yet.
My daughter has been in remission for a year but has been put on the extremely vulnerable list, being a kid makes her theoretically safer, but having already had a cytokine storm means this virus could theoretically cause another.
The immune system is one of the least understood parts of the human body. Clinicians and researchers are developing enormous banks of knowledge, but there is still so much more to learn.