You're doing too much single cause analysis there.
Why would you expect a Christmas spike in an area which is already in a decline caused by herd immunity? You'd expect a mild change to the rate of decline.
Christmas spikes would be in the areas which have not yet peaked.
You've gone to the other extreme. If you had true herd immunity, we wouldn't need a vaccine. But I think we agree....it's a factor that contributes....but it's not the sole driver (which is where we started where you blamed socialization). If you remove the socialization, you still get basically what happened. f you remove the X mas travel, you still get basically what happened.
p.s. please explain the N....makes no sense if you are a data guy.