COVID's severity is because it's making a tradeoff between the time it takes the immune system to destroy it, vs the need to get a host walking around and socializing while breathing it out onto new hosts. The lethality is a side-effect of its replication strategy.
Unchecked, they would just exponentially destroy your tissue.
Cytokine storm responses are a particular kind of immune system over-reaction that can make relatively benign viruses much more dangerous to a healthy host then a more immunologically suppressed host, but that's not the behaviour of COVID-19 which is showing pretty standard lethality profiles.
> Controlling for differences across studies, secondary attack rates were higher in households from symptomatic index cases than asymptomatic index cases, to adult contacts than to child contacts, to spouses than to other family contacts, and in households with 1 contact than households with 3 or more contacts.
It acknowledges asymptomatic spread.
Of course, there's questions of degree.