Think back to early November 2021. People thought "Ok, Delta was bad, but that is probably it now."
THEN OMICRON HIT.
What makes everyone so totally confident, in November 2022, that Omicron was the last major new variant?
THEN OMICRON HIT.
What makes everyone so totally confident, in November 2022, that Omicron was the last major new variant?
I will remind everyone that there are two major ways that new variants evolve: within individual hosts with persistent infections, and among hosts at the level of the host population. The past major new variants (ones that got Greek letters) probably evolved within hosts.
Evolution within hosts can be for different traits than among hosts. Within a host, it's a matter of competition for how well cells are infected, how quickly replication occurs, how many copies get made, etc., relative to different versions of the virus in the same individual.
Among hosts, it's about getting into new hosts. That means transmissibility (how much gets breathed out, how long it hangs in the air, how far it travels, how easily it infects someone breathing it in, etc.) and/or -- especially now -- escaping existing host immunity.
The Omicron variant soup is evolving among hosts in the human population and is about immune escape. BUT, there are at least four ways a major, new, highly divergent variant could evolve.
1) Lots of change within an Omicron lineage. They're evolving very quickly, and I talked about how rates of evolution can lead to major divergence here:
https://twitter.com/TRyanGregory/status/1587862841704398848?s=20&t=qxjg9iWKDa7dpKMNzomq5g
2) Within-host evolution in someone with a persistent infection with an older variant (say, Delta or even the wild type).
pubmed.ncbi.nlm.nih.gov/33922936/
pubmed.ncbi.nlm.nih.gov/33922936/
3) Recombination between an Omicron lineage and another variant within someone with a persistent infection and a co-infection.
Here's a superb thread about how that works:
Here's a superb thread about how that works:
https://twitter.com/firefoxx66/status/1587762412110974976?s=20&t=qxjg9iWKDa7dpKMNzomq5g
4) Ping-pong zoonosis, in which the virus moves into a non-human species, evolves in a different direction there, then moves back into humans. There are *many* other species that can be infected with SARS-CoV-2, including many that we encounter often.
pnas.org/doi/10.1073/pn…
pnas.org/doi/10.1073/pn…
As always, I will reiterate that the way to slow down variant evolution is to have fewer viruses replicating -- that is, mitigate transmission.
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