Post #1253: COVID-19 trend to 9/14/2021, not according to plan.

Posted on September 15, 2021


The Labor Day holiday scrambled the data on new U.S. COVID-19 cases.  Today, that should all have been sorted out, and we should get a clear look at the trend.

Unfortunately, the trend looks ugly.  Today the U.S. stands at 47.3 new COVID-19 cases / 100K / day, up two from two days ago. The case count is right where it was one week ago.

That’s not such a dramatic break-out in logs.  Plotting it in natural units makes it clearer.

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 9/15/2021, from”  The NY Times U.S. tracking page may be found at

The flat part on the end of the curve is an artifact of my fix for the Labor Day data reporting.  But the end of the curve should be in the correct place.

For the past month, the pandemic has behaved in a fairly regular fashion.  The peak of the Delta wave had not materially deviated from the prediction I made on 8/13/2021.

That now seems to have changed.  I guess it’s still possible that this is some impossible-to-fathom fallout from case reporting around Labor Day.  But I doubt it.  Note that, in hindsight, the curve started to flatten out several days ago, that is, before yesterday’s data artifact from Labor Day reporting.

Assuming this is a real change, the question is, what has changed?

One option is that this is all random, and there is no rhyme or reason to it.

A second option is schools.  The timing of it — two weeks into the month of September — suggests this might be fallout from back-to-school.  Unfortunately, there is no timely U.S. database showing COVID-19 cases by age, so there is no obvious way even to take a look at that, other than by looking at whatever states do publish the case counts by age.

A third alternative is that this might be the start of the winter wave.  Seems a little early and a little abrupt for that.

So, since all I can do is guess, I’m guessing this latest change is fallout from the return to school.  As I recall, last year, few places had in-person K-12 schooling.  We wouldn’t have seen a back-to-school bump last year.  But this year, everybody is doing in-person K-12 schooling.  Arguably, that would explain why this year’s pattern differs from last year’s pattern.

In Virginia, where I can get the data by age, that doesn’t appear to to be the case.  There’s no surge of cases among the school-aged population.  But Virginia hasn’t yet shown that upturn in total cases, either.

Here’s my analysis of Virginia K-12 schools, updated to today’s data:

Source:  Calculated from case counts by age from the Virginia Department of Health.

Here’s the U.S. South Atlantic region, with Virginia in black:

Conclusion:  This is purely speculation on my part, as I can’t get my hands on data that would give me a clue one way or the other.  But it looks as if, this year, we may see a fall wave, possibly due to return to school.  It’ll be a while yet before that’s perfectly clear.