My only experience was in 1977 at, say, BSL minus 2 or so ^_^ (e.g. mouth pipetted non-pathogenic E. Coli K-12, something that horrified a friend a decade later), but reviewing the Wikipedia entry the threshold for going from 3 to the rare and expensive 4 is said to be treatablity. It think it's a stretch to say pandemic flu is "treatable"....
Also, WWI had significant impact on yhe diseases evolution. In peacetime mild strains spread better as overly sick people tend to stay home, but you have little choices in trench warfare.
PS: Actual fatality rates are hard to pin down as Aspirin poisoning was a significant issue.
I wasn't aware cytokine storms had moved past the hypothesis stage, but per Wikipedia it sounds like it. Still, treatment of them at scale....
And the other thing is how do you define treatment? Besides Tamiflu and the other neuraminidase inhibitor currently marketed, who's efficacy is very questionable, treatment isn't as I normally define it, it's entirely supportive, including of course zapping secondary bacterial infections.
The 1918 pandemic aspirin poisoning hypothesis is interesting, but I think highly speculative, given all the places zapped that had limited if any aspirin availability, or so I assume for all those Pacific islands: https://en.wikipedia.org/wiki/1918_pandemic_influenza#Devast...
I'm loathe to comment on typos, but in this case do you mean "very" or something about it varying?