Post #1255: COVID-19 trend to 9/15/2021

Posted on September 16, 2021

New COVID-19 cases are up 3% over the past seven days.  But we’re still 4% below the 9/1/2021 peak of the Delta wave.  Currently, we’re at 47.3 new cases / 100K / day, down from a revised 47.9 cases yesterday.   The overwhelming majority of states showed an increase in new COVID-19 cases over the past seven days.

Here’s my regional graph in logs and natural units.


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 9/16/2021, from”  The NY Times U.S. tracking page may be found at

In the log graph, it appears that the nice orderly structure of the Delta wave is starting to break down.  For the past month, the regional trends formed a set of (near) parallel lines.  That is, the regions had similar rates of growth in new COVID-19 cases.  That’s starting to get lost as you get to the right side of the chart.

If we compare the first and second years of the U.S. pandemic (below, graph starts April 1 of each year), this last little episode just looks like a post-holiday blip.  E.g., the wiggles in last year’s winter wave (tallest part of the blue line)  were due to Thanksgiving and Christmas/New Year holidays, respectively.

That problem with that hypothesis is that Labor Day is nowhere near as big a holiday as Thanksgiving.  Which is what appears to be case on the chart above — the “hook” on the end of the red line is about as deep as the first wiggle on last year’s winter wave.  On the other hand, data reporting was more continuous last year.  States weren’t taking the weekends off, as is now commonly the case.  Maybe a minor holiday, combined with sporadic data reporting, can produce that much of a disruption in the numbers now.

But the bottom line is that I can’t find any solid evidence, so far, that this latest change is being driven by anything in particular.

There is no systematic data source that lets me look at cases by age in a timely fashion, so the idea that this latest change is due to schools can’t be tested.

And as for the onset of the winter wave, it’s just too soon to tell.  Growth in daily new COVID-19 cases has definitely shifted into the northern half of the country, as you can see below:

Map courtesy of

That's a lot clearer if I simplify the map to just three colors, below.  Now, with the exception of Louisiana and Illinois, the separation of the states is quite stark.

Map courtesy of

The upshot is that I've gone from having a pretty solid prediction for the last month, to being totally lost as of today.

One option is that this is just a post-holiday disruption of testing and reporting, as we saw happen last year.  But it's grossly disproportionate to the size of the holiday, compared to last year.  And the regional lines are starting to get disorderly, which is what has happened in the past at an inflection point in the curve.

A second option is that this is due to school re-opening.  But there's no hard data yet, on a national level.  All I can get is newspaper reporting, and the occasional state where cases are reported by age.  In Virginia, where I can get the data, there appears to be no upsurge in cases for school-aged children.

Yet a third option is that this is the start of the winter wave.  New-case growth has shifted to the northern half of the country, as it did for last year's winter wave.  But it's at least a month too soon for that to start happening, compared to last year.  On the other hand, Delta is substantially easier to spread than the native variant that was prevalent last year.  So maybe that shifts the timing.

We'll just have to wait and see.