Why would "natural immunity" kick in this suddenly and effectively?
> Behavior didn’t suddenly improve over the holidays; Americans traveled more over Christmas than they had since March.
Yes, and then after the holidays they stayed home, so one or two weeks after the traveling seasons there were much fewer infections. I don't see why this explanation is dismissed.
Anyway, it's not so long until April, so we'll see soon if "Covid will be mostly gone by April". ("By" appears vague, but my understanding is that refers more to "at the beginning of April" rather than "at the end of April", could Americans confirm?) Of course even if it does disappear as suddenly as this person predicts, we might not be able to tell whether it was due to vaccination.