flu mortality is about 0.1%
though, like Covid, varies by age group/pre-existing.
Also varies by strain/year
The parent's point is about the metric of "death rate for severe cases", which is ambiguous depending on how you define severe. You could pick a definition of severe such that the death rate is 100%, but that wouldn't be super useful.
The running trial gives the definitions, it's not ambigous as you wouldn't be able to run a Phase III trial without it.