Post #1172: U.S. COVID-19 fifth wave takes off.

Posted on July 9, 2021


The average number of new COVID-19 cases per day remains low for the U.S. as a whole.  That said, the trend is up, and it’s now showing a fairly rapid rate of growth.  Daily new COVID-19 cases rose 22 percent over the past seven days, as shown on the log-scale graph below.  In hindsight, we can date the end of the U.S. fourth wave and the start of the fifth wave to 6/20/2021.

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 7/9/2021, from”  The NY Times U.S. tracking page may be found at

This fifth U.S. is widely attributed to the emergence of the Delta (formerly Indian) variant of COVID-19.  And yet, except for the outbreak in southwestern Missouri, it’s hard to see that in the data.  Here are the top eight states, in terms of incidence of Delta variant, according to the most recent data from the CDC:

That said, this graph may be misleading because it looks at the at the number of new cases per day, not the growth trend.  All we can tell from this graph is that the only state where there’s a notable outbreak of COVID, right now, is Missouri. 

By contrast, if we graph that with a log scale, below — so that the slope of the line is now the growth rate — we can see that almost all of those lines have turned upward, and several have some fairly steep slopes.

If we take all 20 states where CDC has estimated the prevalence of Delta, and plot against the last month’s increase in cases, it does show a fairly large and statistically significant positive correlation.

The upshot is that, while the picture isn’t crystal clear, and while there’s really only one area seeing crisis levels of hospital admissions from Delta, there is some modest evidence that the uptick in cases across states is systematically related to the level of Delta in a community.

Possibly the picture is unclear because we still don’t have good estimates of the incidence of Delta.  The CDC data on which the graph above is based typically reflect just a few hundred COVID-19 samples per state.

Possibly the picture is being muddled by seasonality.  We had an outbreak in 2020, at this time of year, when none of these new variants were around.  That was a wave centered on the hot-climate, high-air-conditioning-use states.

We should not fall into the trap of thinking that there can be only one cause.  It’s entirely possible that the same seasonality that gave us the second wave of COVID-19 is once again at work.  And so what we’re seeing could be a mix of that seasonality and the impact of the more-infectious Delta variant.

In fact, when I correlate the recent trend with typical summertime temperature by state (my proxy for typical time spent in air conditioned spaces), I find a fairly nice-looking graph.  And when I put both factors into a regression (not shown), the case for seasonality as a cause of the current uptick is just as strong as the case for the spread of the Delta variant.

Finally, it almost goes without saying that we owe this wave of the pandemic to people who refuse to get vaccinated.  While vaccination is not the only way to acquire immunity, it’s the surest.  And so, if we plot recent month’s trend against the fraction of the population that has not been vaccinated, we get the expected result.  The greater the fraction not fully vaccinated, the higher the growth in new cases.  By eye, this factor is at least as strong as the other two.

Well, no matter how you slice it, case counts are rising again.  So far, there’s only one small region with crisis levels of hospitalizations.  Outside of that region, you could make equally strong cases that this latest rise is driven by the Delta variant, or that it’s just seasonality — a repeat of last year’s second wave.  Plausibly, the answer is that it’s a combination of the two.  Whatever the reason, we can chalk this fifth wave up to the significant portion of the population that refused to get vaccinated.