Post #1205: Slowing growth in COVID-19 cases, but no clear idea why.

Posted on August 6, 2021


The U.S. reached 31 new COVID-19 cases per 100K population per day.  New COVID-19 case growth is definitely slowing, with the most recent seven-day increase down to just 40%.

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 8/6/2021, from”  The NY Times U.S. tracking page may be found at

Above, the slowdown is now clearly visible.  On that log-scale graph, the line is  curving a bit toward the end.

That’s more obvious if you simply plot the seven-day growth rate over time.    Here’s the graph of the trailing seven-day growth rate.  The blue line is the percentage change, and it continues to fall:

As to why this is happening, well, it surely doesn’t appear to be due to anything we’re doing to protect ourselves.  As I showed yesterday, mask use is virtually unchanged.  Vaccinations are up a bit, but that won’t affect the current growth in new cases.

The only thing I note is that there’s a strong negative correlation between the number of cases / 100K / day by state, and the growth in that figure over time.

In other words, maybe this phase of the pandemic is starting to peter out in the states that have been hardest-hit so far.  That’s the normal dymamics of an epidemic.  At some point, you start to run out of people who are out and about, taking no precautions, and still uninfected.   It’s that “running out of people” effect that gives an epidemic that classic sigmoid-curve shape when total cumulative cases are plotted over time.

If that’s true, that’s good news.  If I’ve diagnosed that right, we’ll soon see those states with the highest new-case rates start to peak.  Maybe that’s starting to happen, but it’s not crystal clear yet.

Is that group of curves qualitatively different from this group below?  I think so.  There appears to be some downward curvature for the top-ten states above.  I see no such curvature (reduction in growth rate) for the bottom ten states shown below.