Post #1329: COVID-19 trend to 11/23/2021, slow winter wave

Posted on November 24, 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 11/24/2021, from”  The NY Times U.S. tracking page may be found at

Case counts are rising slowly, and are rising in ever region except the Pacific states.

Tomorrow we begin our holiday season.  I’ve set up the second graph below so that I can track case counts relative to what happened during last year’s holidays.  Thanksgiving is one day earlier this year.

Up to now, I’ve been characterizing this year’s winter wave as “late”, compared to last year, likely owing to warm October weather in much of the country.  Best guess, based on mean October temperatures for the U.S., and the typical Fall month-to-month rate of temperature decrease, maybe two or three weeks late.

That may be true, but after this past week of slow growth, that’s starting to wear a little thin / stretch a little far / pick your own metaphor, as a complete description of what’s happening.

If the 2021 winter wave were a duplicate of 2020, just a couple of weeks later, we’d see the orange line above eventually paralleling the blue line.  Instead, ,that gap between last year’s line and this year’s line continues to grow.

Restated, either this year’s winter wave is much later than last year’s, or we have a generally lower rate of growth in cases this year, compared to last year.

We also have a more-or-less a complete lack of a winter wave in three states that were leaders of the winter wave in the Mountain region last year (ID, MT, WY).

Now that I look at it, the three states that led the Midwest region in last year’s winter wave have also mostly failed to get with the program this year.  Shown below.  It’s not as clear-cut as the leaders of the Mountain states — there is a slight upward trend in two of the three — but it is nevertheless a strong contrast to last year’s experience.

I’m not sure what to make of all that, yet, but given that it’s the holidays, I’ll try to end on an upbeat note.

Maybe we are due for a repeat of last year’s wave.  The same as much of Europe is suffering through right now. Just a bit later.

On the other hand, owing to our exceptionally high rate of infection in the past, and to a middle-of-the-road vaccination rate, maybe immunity in the population is now high enough to allow is to avoid another awful winter wave.

(Awful?  People forget.  For nearly the entire month of January 2021, we had an average of more than 3000 COVID-19 deaths per day.  As a result, for that period, the total U.S. death rate increased by more than one-third above the norm.)

Near as I can tell, we ought to be pretty close to “herd immunity”, for something as infectious as the Delta variant of COVID-19.  That’s defined as having enough immunity in the population (via infection or vaccination) to be able to suppress the spread of a virus with basic reproduction rate (R-nought) of 5.0.  (Where the average infected person would go on to infect five others, absent any interventions or immunity).  The table below shows 77%, where we’d need a value of 80% or more to suppress spread of a such a virus with no other measures taken (e.g., without wearing masks).

I have to make a lot of assumptions in that calculation, as it depends on a lot of things that cannot be observed.  For example, it doesn’t account well for the rate at which immunity fades over time.  It doesn’t account for the potential for re-infections (Post #1326).  And so on.

But I think this is enough to show that we’re in the ballpark.  Right now, infectiousness of Delta is rising owing to changing weather conditions.  So it would not be a surprise to have some winter wave.  But we ought to be close enough to herd immunity now that maybe we won’t have much of a winter wave.

Let me put it this way.  Let’s assume Spring is going to occur at roughly the same time this year as it did last year.  At the minimum, the heart of this year’s winter wave will be shorter than last year, reducing the total he total time we’ll spend at those very high winter infection rates.  And maybe we have enough immunity that the peak of this year’s winter wave will be lower as well.  Further reducing overall impact.

No matter how I slice it, it seems like we’re in for a “smaller” winter wave this year.  Either shorter in duration, or shorter in height, or, ideally, both.

Maybe we can take some inspiration from our northern neighbor.  For sure, if cold weather is the issue, they’ve got that covered.  And, while they have a much higher overall vaccination rate, but we beat them in terms of cumulative infections per capita.  So we’re not that different in terms of total population immunity.

And yet, compared to last year, so far, there’s not much of a winter wave happening there. Whatever it is that’s suppressing the start of the U.S. winter wave, it apparently is not limited to the U.S.