Post #1148: COVID trend to 5/17/2021, unfinished business

Posted on May 18, 2021

The number of new COVID-19 cases per day is now down 56% from the peak of the U.S. fourth wave.  As a whole, the U.S. is now well below 10 new cases / 100,000 / day.

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 5/18/2021, from”  The NY Times U.S. tracking page may be found at

There’s a tendency to attribute that to vaccination, but that’s clearly only part of the story.  For sure, it appears that states with high levels of immunity (via vaccination or prior infection) are seeing the steepest rates of decline.  But that effect appears limited just to the states with the very highest levels of immunity.  Once you get beyond the top 10 or so, the level of immunity doesn’t much see to affect the downward trend.  States at all levels of immunity appear to be trending downward at the same rate.

(The “immunity level” here is my estimate of the total population immune to COVID-19 via infection or vaccination, including all my most recent corrections.  That’s the same analysis by which I estimate that about 60% of the U.S. population is now immune to COVID, Post #1144.)

And when you get right down to it, if you look at Canada over the same time period (left), you’ll find that their cases peaked just about the same time ours did.  And yet, at the time of that peak, Canada was far, far behind the U.S. in terms of total vaccinations.  And Canada still remains substantially behind the U.S. in terms of total vaccinations.


My guess is that the inherent seasonality of coronavirus is playing a role here as well.  Part of what we’re seeing is that it’s the time of year for COVID-19 to go into retreat, just as it did at this time, last year, to end the first U.S. wave of COVID-19.  Just like, say, flu.

My point is, the job isn’t done yet.  Things look pretty good right now.  But only pretty good.  But they don’t look end-of-the-pandemic spectacular.

For sure, we’re never again going to see anything as bad as the third (wintertime) wave, because we just don’t have that many people left who aren’t immune to COVID.  But there’s still roughly 40% of the U.S. population who remain capable of being infected with COVID-19.  And that’s another 20% to go before we reach what most scientists are talking about for herd immunity now — 80% of the population.

Slacking off

The use of masks in public places had been declining for months before the CDC made its pronouncement last week.  That pronouncement being being that:

Fully vaccinated people can resume activities without wearing a mask or physically distancing, except where required by federal, state, local, tribal, or territorial laws, rules, and regulations, including local business and workplace guidance.

Source:  CDC

Here are two snapshots of mask use, based on a simple (but large-scale) Facebook survey.  That’s early February on top, early May on the bottom.  Darker means higher rates of mask use.  That trend had been going on for months, and accelerating of late.

Source:  Carnegie-Mellon University Delphi Group.

Obviously there’s a lot of local variation.  Around here (Vienna, VA), most but not all private enterprises around here haven’t dropped their mask requirements.  The Home Depot still has a “masks required” sign prominently displayed. Everyone I saw was in compliance.  My local Safeway appears to have taken that down.  And yet, most people I saw were still wearing a mask.

But here’s the funny thing.  The same simple plot that suggests that vaccines appear to be working also shows that mask use appears to be working.  As below.  If I hadn’t labeled them, you’d be hard pressed to tell which was correlation with mask use, and which was the correlation with population immunity.  (And if I “run the regression”, these two factors are almost completely independent and both are statistically significant.)

But the masks are going to continue to come off, regardless.  Which may be fun, but slows the rate at which COVID-19 disappears from the community.

The pace of new vaccinations continues to slow, but has not stopped.  It looks like the elderly are going to top out at around 85% vaccinated.  It’s too soon to tell what fraction of the rest of the U.S. population will be vaccinated.

Source:  CDC

All that does is push off the final end of the pandemic further into the future.  Maybe enough people will get vaccinated that the U.S. as a whole will reach herd immunity, and this pandemic will finally come to an end.  But that’s not a certainty.

In any case, we aren’t done yet.  The extent to which this latest decline in case counts is merely due to seasonality is not clear.  The extent to which we may yet see flare-ups (as air conditioning season hits this summer, and then as winter sets in later this year) is still unknown.  But this is the path we’re on, and we just have to deal with the consequences.