“It became dominant over Delta in Johannesburg (the 26th biggest city on Earth) just two weeks after it was first detected. No other COVID variant had been able to take dominance from Delta, anywhere on Earth. Omicron not only did it, but it did it much faster than any COVID variant has every risen to dominance.”
*****
I don’t understand how one moving in makes the other go away, though.
“I don’t understand how one moving in makes the other go away”
It may well be less the case with Omicron, than it has been before.
So far, infection with any strain of COVID induced very strong immunity against re-infection by the same, any other strain. Whichever strain got to a host first, immunized that host against the other strains.
The less infectious strain would continue spreading, but it would be on a declining rather than growing trend, until it dwindled out. They overlap for some time, as the more infectious improves its growth rate, at the expense of the dwindling rate of the less infectious.
But Omicron is so different, so heavily mutated, that the cross-immunity among strains seems to be weaker (although still stronger than the vaccines seem to be).
The prospect has been raised, that we might even see people who were simultaneously infected with Omicron and Delta - but I have not heard of any actually reported.
Between the very infectious Omicron and Delta however, we are rapidly on the way to getting to Herd Immunity Threshold.
Delta seemed like it would run out by Spring. Omicron looks more like a month or two.