lpm wrote: ↑
Wed Jul 07, 2021 1:26 pm
I think what I'm ultimately struggling with is why the 1918-19 virus was a great global winner, at R=4, and yet the world settled down again to our range of seasonal flus, at say R=1.5. Why should a world beater turn ordinary?
Well, you sort of get ebbs and flows with 'flu, in that most of the time you have not-too-bad seasonal flus running in the background, but then sporadic epidemics of more successful strains. The 1889-90 one, Spanish in 1918-20, Asian in1957/58, HongKong in'68/69, possibly a Russian one in '77-79, then that swine flu one in 2009.
And iirc, 1918 was milder in the early spring wave, fiercer in the autumn wave that year. There's a theory that was because soldiers who were very sick got moved to field hospitals, so that facilitated the spread of more virulent strains - or, if you want to look at it in terms of selection pressure, keeping mild cases on the front gave the virus an incentive to become less mild so it could reach more hosts.*
There's another theory, of course, that a bigger factor was terrible conditions in the trenches and general ill health & poor conditions across Europe at the time.
(*I'm sure I've seen it argued that we created similar pressures with Covid by isolating mild cases at home, but it's hard to stop hospitals becoming a source of transmission.)