We saw the worst of it in the northern states in the winter, and in the southern states in the summer. There’s a very simple explanation for this bifurcation: time spent indoors.

About 50% of the population seems immune to it. The easiest explanation is that they have T-cell immunity from prior coronavirus infections (common colds), and if this is correct, then the immunity should be long lasting as the people who got SARS in 2003 are immune to this year’s Covid.

About 20% of the population of New Dehli has antibodies to it. This number seems to be reproducible in every place that has had a wave go through it.

So now 70% of the population cannot get it.

At that number, it’s basically herd immunity. Sporadic cases may happen, but it’s very unlikely that more big waves will come. We might get small ripples in the northern states in the winter and southern states next summer, but that should be the worst of it.

In my own practice in Arizona, I have seen exactly ZERO cases the last 2 days whereas two weeks ago, I was seeing 10-15 a day.

Disclaimer: I reserve the right to be wrong.

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A simple model of “herd immunity” (as I understand it).

Imagine a room of 50 people none of them are immune to a virus. One person in the corner of the room gets the virus. He will spread it to the 3 closest people. They will each spread it to the 3 closest people to them. Very soon a massive wave will rip through the room infecting everyone.

Instead, imagine a room of 50 people, but now 20 of them are immune to the virus. One person in the corner of the room gets the virus. He spreads it to the people closest to him, but 1 of the 3 is immune. That’s a break in the transmission wave. The virus will still spread through the room, but there will be breaks sporadically, and probably not all of the 30 non-immune people will get it.

Imagine now that 35 of the 50 people are immune. One person in the corner of the room gets the virus. It will be very hard for the virus to infect all 14 remaining people. There will be too many people immune in the path of the spread. It might even be the case that the 3 people closest to the initial person will all be immune and none of the others in the room get infected.

That’s herd immunity: introduction of essentially “circuit breakers” in the path of spread.