I just created a post explaining a new dashboard created and some of the slightly tricky calculations involved. Readers were instructed to skip that post if they did not have technical interest. Nevertheless, I will use the fruit of that labor to present some findings here (and perhaps subsequent posts). Here I will do a quick summary of what I see in US and Canada (used dates 3/15/20 - 12/31/22)
The expectations list is problematic. 2020 waves reversed with seropositivity and experienced illness still ultra-low, 3 - 10% in lockdown countries. In the West only Sweden and Italy were higher https://onlinelibrary.wiley.com/doi/10.1111/vox.13221 - Granted, the dead can't contribute to seropositivity, but deaths were essentially rare vs. population, so the AR among "susceptibles" would be the same as overall AR, with a possible caveat for locales that turned nursing homes into hotspots.
So there's no grounds for asserting that susceptibles have been removed. Presumably only 3 to 10% of them have. There's lots of shoes left to drop.
As such, any gains in fitness / virulence would at best have a .1 discount from prior immunity / removal of susceptibles in most countries. .1 is a rounding error in terms of the possible delta of mutations on fitness / virulence. Given that the "reward" for increased fitness is an additional 9 times as much infection and replication as achieved in the first wave, i.e. the conquest of that remaining 90% of hosts, there's no reason to expect any reduction in deaths in future waves per se. And this just repeats for all additional waves up to Delta, because they keep ending with not much movement on seropositivity.
Only in the Omicron era can you start to include accumulated natural immunity as a serious factor in expected/observed epidemiology/evolution. Hence why it is only recently that there has been a lot of actual immune escape mutations, whereas the 2020 VOCs had one or two escape mutations to their name at most.
Also - it may be useful to link to the previous post
The expectations list is problematic. 2020 waves reversed with seropositivity and experienced illness still ultra-low, 3 - 10% in lockdown countries. In the West only Sweden and Italy were higher https://onlinelibrary.wiley.com/doi/10.1111/vox.13221 - Granted, the dead can't contribute to seropositivity, but deaths were essentially rare vs. population, so the AR among "susceptibles" would be the same as overall AR, with a possible caveat for locales that turned nursing homes into hotspots.
So there's no grounds for asserting that susceptibles have been removed. Presumably only 3 to 10% of them have. There's lots of shoes left to drop.
As such, any gains in fitness / virulence would at best have a .1 discount from prior immunity / removal of susceptibles in most countries. .1 is a rounding error in terms of the possible delta of mutations on fitness / virulence. Given that the "reward" for increased fitness is an additional 9 times as much infection and replication as achieved in the first wave, i.e. the conquest of that remaining 90% of hosts, there's no reason to expect any reduction in deaths in future waves per se. And this just repeats for all additional waves up to Delta, because they keep ending with not much movement on seropositivity.
Only in the Omicron era can you start to include accumulated natural immunity as a serious factor in expected/observed epidemiology/evolution. Hence why it is only recently that there has been a lot of actual immune escape mutations, whereas the 2020 VOCs had one or two escape mutations to their name at most.