Post #1141: New COVID-19 cases cases per day down 21% in the past week.

Posted on May 11, 2021

The end of the U.S. fourth wave of COVID-19 continue to pick up speed.  In the past seven days, the rate of new cases per 100,000 per day decline 21 percent.  In total, there has been a 45 percent reduction in new cases per day since the peak of the U.S. fourth wave last month.

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 5/11/2021, from”  The NY Times U.S. tracking page may be found at

The other bit of good news is that it doesn’t look as if the vaccination rate is going into free-fall.  There’s been a little up-tick.  It’s not much, but it’s better than nothing.

Source: U.S. CDC

Seasonality, again.

I would love to attribute this decline in new cases to vaccination, but its not clear that vaccination is the only driver here.  It’s starting to look as if there may be a fairly large component of seasonality as well.  (Which is more-or-less another way of saying that we’re getting lucky, right now, and that this is just the time of year when this coronavirus has a harder time circulating in the population.)

First, as you can see from the first graph above, there are strong regional differences in trends.  But regional differences in vaccination rates are modest at best.  You can’t just blithely say that vaccination is the root cause of the decline, when vaccination appears to be doing next-to-nothing for the U.S. Mountain region.

The strong geographic clustering of trends suggests that maybe the regional climate has an influence.  The entire North American east coast has fully entered into spring-like weather.  By contrast, in (say) Denver, Colorado, nighttime temperatures are still flirting with freezing.

Second, below, Canada has struggled to get its population vaccinated, and has a far lower vaccination rate than the U.S.  And yet, Canada’s  most recent COVID-19 wave peaked within days of ours, and it too is now showing an accelerating rate of decline in new cases.  So, once again, Canada’s changes are synchronous with ours, as they have been through much of the pandemic.  This suggests some sort of common influence that passes beyond national borders.

Finally, let’s not forget that the very first U.S. wave of COVID-19 — the one largely confined  to NY City and parts of the U.S. West Coast — peaked on 4/10/2020, and declined throughout the rest of April and into May.  While we attributed that to lockdowns and such, the fact is, that attribution is just a guess.  Coincidence or not, we should not lose sight of the fact that the timing of the U.S. fourth wave is a close match for the timing of the U.S. first wave. 

Interestingly, if COVID-19 is still in circulation this summer, this whole “seasonality” concept will face an acid test just two months from now.  In early July 2020 we saw a wave of new cases in hot-climate states, plausibly attributed to air conditioning use.  This year, if we see rates begin to rise in the hot-climate parts of the country, in early July, we’ll know that history is repeating itself.

All of that arguably relates to indoor air humidity as a modulator of spread of COVID-19.  I went over the arguments for that in Post #894.

The final reason to doubt that this is the result of vaccination alone is that there’s no clear link between overall population immunity and current case trend.  There are large differences across the states in the total population presumed to be immune, if you combine both vaccinated individuals and those who have already been infected with COVID-19.  Despite that, the states with the apparent highest rates of immunity are not, in any sense, leading the trend downward.  They are a mixed bag.

That said, we are seeing some impressively steep declines in new case counts, in some states.  I don’t know if there’s any particular value in highlighting that, but both New Jersey and the District of Columbia saw more than a 40% decline in daily new cases in the past week.  In addition, we now have 18 states where the new case rate is in the single digits (below 10 / 100,000/ day).

For whatever reason, the COVID-19 situation in the U.S. is improving rapidly, and the pace of improvement keeps accelerating.  Maybe this is a case of not looking a gift horse in the mouth.