Post #1147: COVID-19 trend to 5/14/2021.

Posted on May 15, 2021

For the U.S. as a whole, new COVID-19 cases per day are now less than half what they were at the peak of the U.S. fourth wave.

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 5/15/2021, from”  The NY Times U.S. tracking page may be found at

I’d be willing to bet that at least some of that decline is due to the natural seasonality of COVID-19.  But, as I noted in Post #1145, states with the highest levels of immunity to COVID-19 (via prior infections and vaccinations) are showing the steepest rates of decline, on average.

There are two easy ways to show that, contrasting the states with highest and lowest levels of population immunity to COVID-19.

The two charts below show the top ten and bottom ten states in terms of (my estimated) population immunity to COVID-19.

The steeply-curved blue line on the top graph is New Jersey, where the  number of new COVID-19 cases per day has fallen by more than 80% in the past two weeks.  They are now below 5 new cases / 100,000 /day.   (And the near-vertical line on the second graph is Alabama, where a data reporting issue is putting yet another “speed bump” into the data.)

A second way to show that is to contrast the states divided into fifths (“quintiles”) based on (my estimate of) population immunity to COVID-19.  If I focus on just the past two weeks, and start them all at the same arbitrary point, there’s clearly a widening gap between the trends for the top and bottom ten states.

Interestingly, it’s really only the top 10 states that are breaking away from the pack.  My estimate is that in those states, 80% or more of the population is now immune to COVID-19.  That could be some weak evidence that the level at which “herd immunity” starts to kick in is somewhere around that 80 percent mark.