Post #1260: COVID-19 trend to 9/20/2021, another big drop in cases.

Posted on September 21, 2021


The U.S. is now 17% below the 9/1/2021 peak of new COVID-19 cases per 100,000 per day.  Currently we are averaging 42.6 new cases / 100K / day, down from 46.1 last Friday.

Only one state — Alaska — is over 100 new cases / 100K / 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 9/21/2021, from”  The NY Times U.S. tracking page may be found at

First, the sharp break at the right end of the line is an artifact of my methods.  The “true” curve, if we could have seen the data continuously over the weekend, almost certainly would have made a smoother transition to that final endpoint.

The end of the line is in the right place, based on the data.  But the two days prior to that contain a lot of data that I generated, to gap-fill for the states that don’t report data over the weekend.  My gap-fill is backward-looking — I use the most recent seven-day moving average for each non-reporting state.  If, in reality, there is a sharp downward change in the trend, my backward-looking method overshoots the real numbers.  And that gets straightened out all at once, on the day when all those states report their next set of numbers.  Which is the last point on that line.

That said,  my “blocky” curve ends up in the right place.  However you slice it,  there’s been a sharp downward trend in daily new cases.

Putting the methods aside, I think this drop is real, and not some artifact of the sketchy data reporting.  I say that because sharp drops in the (true, reported) new case count occurred across a large number of states.  That’s just another one of those weird things about this pandemic.  It’s like somebody rang a bell over the weekend and said, it’s time for the Delta wave to finish up now.

That’s not to imply that it actually is over.  There are still some states showing rapid increases.  But the preponderance of states showed significant declines over the weekend.  Below, you see the top and bottom five states for new case growth over the last seven days.  At this point, the most rapid case growth has shifted out of the South entirely.

Finally, just a reminder that the state-level data really don’t show you what’s going on, as the disease is highly concentrated among the unvaccinated portion of the population.  Here’s the latest weekly data from Virginia.

Source:  Virginia Department of Health COVID-19 dashboard.  Notes added in red.

The vaccinated population of Virginia has a new-case rate lower than any state.  By contrast, if the the un-vaccinated population of Virginia were its own state, it would rank fourth in the nation.