Post #1321: William and Mary COVID-19 trend to 11/12/2021 — one more case this week, and the U.S. winter wave in the South Atlantic states.

Posted on November 13, 2021

Source:  Calculated from the William and Mary COVID-19 dashboard, accessed 11/13/2021

2021 Winter wave in brief.

Things look great at William and Mary, but for the U.S. as a whole, it looks like the winter wave has started.

Last year, the winter wave started in the Midwest states.  Eventually, all areas of the country saw a mid-winter increase in daily new cases.  Last year, the South Atlantic states ran a month or two behind the Midwest.  The Midwest peaked around Thanksgiving, the South Atlantic did not peak until mid-January 2021.

This year, new cases turned upward in the Midwest around Halloween. And now, about six weeks later, it looks like cases are just starting to rise in the South Atlantic states.

That’s later than last year, plausibly due to much warmer weather in the middle of the country this year, compared to last.  Below are maps for October 2020 and October 2021.  Last year (top) was much colder than normal in the middle of the country, this year (bottom) has been much warmer than normal.

In any case, below, that little upturn in cases at the right edge of the graph may not look like much.  But it’s more-or-less right on time to be the start of the winter wave in Virginia this year.  The fact that several adjacent states (NC, VA, WV, MD, DE) all show the same pattern suggests that it’s weather-related, and not just a statistical fluke in (say) Virginia’s case counts.

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 11/13/2021, from”  The NY Times U.S. tracking page may be found at

My best guess is that the little upturn above is, in fact, the start of the winter wave in Virginia.  At this point, there’s no telling where that’s going to go.

I guess I’ll mark the start of the U.S. winter wave at 10/25/2021.  That’s when the U.S. daily new case count reached that most recent minimum.  In the past seven days, new case counts rose 9% for the U.S. as a whole, and were  rising in all regions except the Pacific region.

You’d think we wouldn’t have a winter wave this year, with all the people who’ve been vaccinated or have recovered from infection.  But the example of Europe pretty clearly shows us that a winter wave is possible.

When I look at the numbers, I estimate (guess) that we’re in roughly the same situation as we were last winter, in terms of the ability of the virus to spread (the “R-effective”).

Compared to last winter, we have:

  • Higher levels of population immunity (best guess 71% now versus about 10% in October 2020).
  • A far more infectious variant of COVID-19 (best guess, R-nought of 5 for Delta, versus maybe 2.5 for last year’s native strain, meaning that without any preventive measures, each person infected with the Delta variant would have infected an average of five others.)
  • Much lower levels of COVID-19 hygiene (no restrictions on public gatherings, mask-wearing down from 95% in the middle of last year’s winter wave to 60% now).

When I run that through a crude formula, including my own estimate for the impact of peak COVID-19 hygiene, and an assumption that current COVID-19 hygiene is, on net, half as effective, I come up with:

  • R-effective last winter 1.035
  • R-effective this winter 1.075

I wouldn’t put a lot of faith in either number.  I’m just saying that the magnitudes of these effects are in the ballpark of cancelling each other out.  It’s entirely possible that we’ll have a winter wave despite having 59% of the population fully immunized, and a further large percentage with some immunity due to prior infection.

Source:  CDC COVID data tracker.

To recap:  Best guess, that level of vaccination, plus all the prior infections, is just about enough immunity to offset the greater infectiousness of Delta and the reduction in COVID-19 hygiene.

Finally, FWIW, in the popular press you’ll hear the rise in cases attributed to people spending more time indoors.  Near as I can tell, that’s more-or-less nonsense.  In the modern world, there’s maybe one hour’s difference in total indoor time, winter-versus-summer, for the average adult.  There’s far more evidence to suggest that flu season is a result of dry indoor air.  Low relative humidity reduces the body’s ability to clear way inhaled pathogens (“mucocilliary clearance”) and otherwise reduces the effectiveness of the immune system at fighting respiratory illnesses.  I lay out the evidence for various hypotheses regarding why we have a winter flu (and now, coronavirus) season in Post #894 about relative humidity and flu.