Post #1302: COVID-19 trend to 10/20/2021 — no change.

Posted on October 21, 2021

U.S. new cases continue to drop, and we’re now 54% below the 9/`1/2021 peak of the Delta wave.   Today the rate stands at 23.7 new cases per 100K per day.

Data source for this and other graphs of new case counts:  Calculated from The New York Times. (2021). Coronavirus (Covid-19) Data in the United States. Retrieved 10/21/2021, from”  The NY Times U.S. tracking page may be found at

Other than Alaska, the entire U.S. winter wave so far consists of a trio of Mountain states with persistently high COVID-19 new case rates:    Montana, Idaho, and Wyoming.


But that’s it.  That, so far, is the full extent of the U.S. winter wave.

Oddly, these three states exhibit an entirely new phenomenon in the U.S. pandemic, a “flat-topped peak” for want of a better term.

Up to now, COVID-19 waves resulted in sharp peaks in state-level new case rates rates.  Within each wave, the states with the highest new case rates would hit a peak, then almost immediately begin to recede.

Below is an illustration of that with the three Southern states that lead the Delta wave.  Until now, that’s been the cannonical shape of a state-level COVID-19 wave.  And, in fact, I used that regularity to predict the peak of the Delta wave.

Contrast the sharp peaks above with the three Mountain states below.  Wyoming, for example, has been seeing 70+ new cases per day for most of the past two months.

And now that I see that, the more I see it throughout the data, for the non-peak states.  If I look closely, I can make the case that the Delta wave seems to be hanging on in many U.S. northern states.  It may not be hanging on at high new-case rates, but the peak rate is persisting.

Contrast the U.S. Northeast region (on a log scale, which tends to compress any differences across states):


With the U.S. South Central region, again on a log scale:

By eye, new-case rates haven’t changed much in the typical Northeast state since mid-August.  The story is much the same in the U.S. Midwest and Mountain states (which is why the regional graph at the top of the post looks as it does.)

And so, while we’re not having a winter wave (yet), the U.S. Delta wave appears unusually persistent in U.S. northern and mountain states.  But not in the South and West.

Maybe that’s how this year’s winter wave will play out.  Maybe we’ve gotten enough immunity via vaccination and prior infection that the typical U.S. state isn’t going to catch fire as it did last winter.  But we don’t yet have enough immunity to get the virus out of general circulation.

I don’t know.  As of today, I’m just pointing out that this appears to be a change from prior behavior.  Before the Delta wave, a persistently high rate of new COVID-19 cases has not been typical of the U.S. experience.  But that seems to be changing as we head into the 2021 winter.