Both the worst-case and the best-case scenario are on the table. Only time will tell what happens. As you seem to be focusing on the worst-case scenario I will talk about the best-case scenario. The best-case scenario is that the human immune system is getting used to corona with even vaccinated people spreading the delta variant with little chance of getting very ill. This will turn covid-19 into something like the common cold or possibly the flu. Who knows what will actually happen? Both my best-case scenario and your worst-case scenario can happen. I think, though, that one should not only focus on the adaptability of the virus. The immune system also has been doing defense against various pathogens for many millions of years.
The likely scenario is that it turns into cold/flu.
Everyone either gets the vaccine or the virus and we are all mostly immune. It mutates a little and once it mutates just enough to spread substantially everyone who gets it gets immune. Only a fraction of the population will get any particular mutation.
I remember having the debate that herd immunity has never happened without a vaccine. Which is just BS. Every disease throughout history has eventually reached equilibrium with humans which is essentially herd immunity.
Most mutations are point mutations. Once enough point mutations accumulate that give the virus superior ability to spread, it will. It is unlikely that any point mutation will suddenly make the virus orders of magnitude more deadly.
> Every disease throughout history has eventually reached equilibrium with humans which is essentially herd immunity.
Not a biologist here but can you elaborate on this? Do you consider all circulating diseases such as HIV, yellow fever, malaria, hepatitis etc. having reached "equilibrium"?
Well, humanity still exists, therefore no disease wiped out all humans. That's a form of equilibrium, and it's a lot better than some other species.
And now we have decent medicine, we can properly turn the tables on diseases. mRNA vaccines are literally sci-fi stuff, and now they're here! Antibiotics weren't even in sci-fi, and we've got them (though they're no longer panaceas).
You don't understand what "herd immunity" means. Herd immunity means eliminating a disease within a community ("herd"). It does not mean endemic case incidence with socially tolerable outcomes, or whatever loosey-goosey idea you mean by "equilibrium."
Herd immunity means the contagion is eliminated. It has never happened worldwide without a vaccine, and I'm unaware of any virus where it has been sustained within a community without a vaccine. How could it? The only paths to immunity are vaccine or exposure, and exposure depends on the virus actually circulating.
At some point this is just arguing semantics, but my understanding is that the herd immunity threshold is defined when Reff < 1. You could have Reff=0.99999999, which counts as herd immunity being reached, but it would take a very long time for the virus to actually disappear. Even if we do momentarily hit Reff < 1 for the delta variant, there could be mutations/waning immunity that causes it to stay around 1 becoming endemic.
No, it is impossible to reach herd immunity if Reff >= 1, but Reff < 1 is not sufficient for herd immunity. Herd immunity also means low enough incidence that a nonimmune person is not at significantly greater risk than an immune person.
Herd immunity would usually be defined as: if a contagion is introduced to a community will it spread uncontrolled or will it die off. Reff < 1 implies it will die off, Reff > 1 implies it will spread. Herd immunity does not mean that the contagion is not present within the community or anything about the relative risk between immune/non-immune community members.
