The wide variation in COVID-19 symptom severity is currently speculative.

“Reports indicate the range of this virus in terms of symptoms is drastically wide, ranging from fully asymptomatic to fatal. And despite earlier reports, there are significant deaths, even among those under 50. What accounts for such variation person to person, especially among otherwise healthy people? Are there other viruses like this? Is it different viral strains or genetics immune system types? There are likely many, many, many different reasons for the varied ways people respond to this virus, but right now, everything still remains speculation.”