Post #1166: The U.S. fifth wave gathers speed, driven more by seasonality than by the Delta variant.

Posted on July 3, 2021


It surely looks like we’re well into the U.S. fifth wave of COVID-19.  Here’s the regional graph looks as of 7/2/2021.  New cases were up 18 percent in the last seven days.  That’s driven in part by a fairly large spike in cases reported by Florida.

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 7/3/2021, from”  The NY Times U.S. tracking page may be found at

For sure, we can thank unvaccinated Americans for this wave.  But beyond that, it’s not exactly clear what’s driving the bulk of it, because we have two things going on in the U.S.:  Spread of the Delta variant, and summertime air conditioning season.

Certainly, the outbreak centering around southwest Missouri can be attributed to the spread of the Delta variant.  And the spread of that more infectious variant is a likely driver for upturns in some other areas.

But it’s also time for a repeat of the summertime wave of COVID-19 in hot climate states.  That wave was attributed to citizens spending more time in indoor, air-conditioned spaces.  My guess is that this is due to low indoor humidity (Post #894), which is a byproduct of heavy air conditioner use.

It’s worth taking a look back at that wave, shown starting July 1 2020.

As you can see, that was a wave of the U.S. Gulf Coast states, with the hot-climate states of Arizona, Nevada, and South Carolina thrown in for good measure.  (Not shown, Georgia was among the states that showed a distinct peak during that period.  So the second wave actually encompassed all the coastal states from Texas through South Carolina, plus Arizona/Nevada.)

Based on the most recent Delta variant estimates from CDC, let me now compare the recent trends in the high-Delta-variant states, relative to the hot-climate states identified from 2020.

By eye, the graphs make a much stronger case for this being mainly a repeat of last year’s summertime wave in hot climate states.  All of the hot-climate states that participated in the U.S. second (2020 summertime) wave are showing upturns in counts (bottom graph).  By contrast, enough of the high-Delta-variant states have such flat trends that the overall graph appears flat at the end, other than for Missouri.

We should not fall into the trap of thinking this must be driven by one or the other.  It can be driver by both, or by neither.  But if I had to parse it out at this point, it looks like the current wave, right now, is driven more by seasonality — a repeat of last summer’s hot-climate wave — than by the spread of the Delta variant.