Post #1214: COVID-19 trend to 8/16/2021, new case growth continues to slow.

Posted on August 17, 2021


There’s another glitch in Florida’s data reporting this week.

This time, I’ve fixed it as best I can.  As a result, compared to what you might see in the newspapers, I don’t show a spike in Florida cases, and I show lower growth in cases for the U.S.

You’ll just have to trust that the numbers I’m reporting are more nearly correct.   Tomorrow, it’ll all be a wash, and everyone’s numbers will be back in sync again.

With that correction, U.S. is at 41.5 new cases / 100K / day, and the seven-day growth in new cases was just 15%. 

And so, growth continues to slow as we approach what should be an early-September peak of the Delta wave.  Unless, per prior post, the Delta wave morphs into the start of the U.S. winter wave.

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 8/17/2021, from”  The NY Times U.S. tracking page may be found at

No matter how you reckon it, growth in new cases is declining.

No matter how you cut it — either highest current case rates versus lowest, or earliest Delta wave start date versus latest — one segment of the country appears to be approaching a peak of the Delta wave, and one is not yet at peak.  With both contrasts, the lines on the left graph are starting to curve, the lines on the right graph are not.

High versus low new cases / 100K / day.

Early versus late Delta wave start date.

Delta and Winter waves in context

Finally, I think I can now start to put the Delta and Winter waves into context by overlapping the first and second years of the pandemic.  I have traditionally tracked the data starting 4/1/2020, so I’m going to start my “pandemic year” on that date.

The graph below shows the first full year of the pandemic in blue, starting 4/1/2020, and the second year of the pandemic so far, in orange, starting 4/1/2021.  The tag end of the orange line, sticking up in the air — that’s us, right now, at this stage of the Delta wave.

As you can see, the current Delta wave is a couple of months later than last year’s summer wave.  If I were to sketch in a likely course of events, assuming a repeat of last year’s winter wave, then the Delta wave will have a distinct peak.  We’ll see new case rates decline for a couple of months.  And then we’ll pick up the upslope of the winter wave.  So here’s how I would sketch it in:

The big question is how severe this year’s winter wave will be, compared to last year.  That’s a great question, and I don’t think anyone can answer it well.  But that won’t stop me from taking a swing at it.

On the one hand, last year only a small fraction of the population had immunity.  Now, a very large fraction of the U.S. population has some degree of immunity to COVID-19, either via vaccines or from prior infection, or both.  And that fraction continues to grow as the Delta wave carries on.

On the other hand, the Delta variant is about twice as infectious as the “native” variant that was circulating in the U.S. during the last winter wave.

Finally, our collective COVID-19 hygiene has deteriorated. 

If I had to throw some numbers at that, I’d start with an estimate that, right now, about 63% of the U.S. population is effectively immune to COVID-19.  That’s from a combination of prior infection and vaccination.  That counts 84% of fully-vaccinated and 54% of partially vaccinated as immune, based on the most recent clinical trials data regarding Pfizer vaccine and Delta variant.  It assumes two unreported infections for every reported infection, and then assumes infections are as effective as vaccines in preventing re-infection (84% effective).  And nets out a presumed overlap between infections and vaccinations.

Another way to say that is that, by my best guess, about 77% of the U.S. population has either had COVID-19 or been vaccinated against it.  Once you adjust for how effective that immunity is against infection, that’s how you get down to that estimated 63% “effectively immune” against COVID.  The 77% partially immune is equivalent to having 63% of the population 100% immune to COVID-19.

Then you have to add in the continuing increase in persons getting vaccinated and infected.  Assume our current rate of about 700K vaccine doses per day (equivalent to 350K fully-vaccinated individuals per day) continues right on up to December 1.  Assume that new infections appear at an average of half the current rate, or about 60K persons per day, and that there are three total infections for every one reported for a total of about 180K new infections per day, including both reported and estimated unreported infections.

Where would we stand, then, as of November 1, 2021, roughly the time I expect the downslope of the Delta wave to hit the upslope of the Winter wave.   That would add about 23M fully-vaccinated individuals and 13.5M infection survivors to the pool.  I project that 69% of the population would be effectively immune to COVID-19 as of November 1, 2021.

Without explaining exactly what I did in the table below, my takeaway is that the severity of this year’s Winter wave depends strongly on behavior.  If we went into full lockdown, we could probably beat it, with that large a fraction of the population immune.  That’s the first number in yellow, below.

But if we do things half-way, then I would expect to see this year’s winter wave be about as bad as last year’s wave.  The last time I did this calculation ( July 10 Post #1173), that half-hygiene number (the second number in yellow) over-estimated the actual peak rate of growth of the Delta wave.  If I adjust these numbers down to reflect that, I’d get a projected peak weekly growth of about 35%, with half-way COVID hygiene.  By contrast, the actual peak weekly growth rate for the 2020 winter wave was about 40%.

My takeaway is that if we continue with half-way COVID-19 hygiene, we should expect the 2021 winter wave to be about as bad as the 2020 winter wave.  Best guess, compared to last year, the vastly higher fraction of the population immune to COVID-19 just about offsets the greater infectiousness of Delta and lower levels of COVID-19 hygiene likely to prevail this winter.

If I had to plan on it, I’d plan on the Delta wave peaking in early September, a few months of declining case counts, then a winter wave that’s about the same as last year.  That’s my best guess.  Substitute a better one if you can find it.

As long as I’m tossing out numbers, here’s the good news.  You see the 77% figure up above?  For what it’s worth, with all the assumptions I had to make to get that number, that’s my best guess as to the fraction of the population that has either had COVID-19 or been vaccinated against it.

I think that’s plausible, given that the CDC estimates that 60% of the entire U.S. population has had at least one shot of vaccine, and that 11% have had a reported case of COVID-19.  Add in some estimate of unreported cases, net out the overlap with the vaccinated, and 77% is at least plausible.

Using the same methods, if this year’s winter wave results in as many infections as last year’s, and we continue to vaccinate 350K people per day, by the end of this winter’s wave, best guess, something like 96% of the U.S. population will either have had COVID-19, or been vaccinated against it.

You have to take that with a large grain of salt, as, among other things, it doesn’t really account for the fact that we can’t yet vaccinate kids under age 12.  Kids under 12 account for almost 15% of the U.S. population.  So maybe the right way to interpret that 96% projection is “almost everybody”.  If the winter wave is as bad this year as it was last year, but the end of it, almost everybody will either have had COVID-19 or had the vaccine.

But just taking it at face value, factoring in an average 84% effectiveness of all that immunity, that means we’ll have just about 80% of the U.S. population effectively immune to COVID-19.  If the R-nought of Delta really is 5.0, and nothing worse comes along, that 80%, plus a bit of residual COVID-19 hygiene, ought to be enough to end the U.S. portion of the pandemic.

To sum it up, my best guess is that after the Delta wave, there’s one more wave to go.  But just one more.  If it shapes up to be about as bad as it was last year, by the time that’s over, almost everybody in the U.S. will either have had COVID-19, or been immunized against it.  And that’s how pandemics end.