Throughout the hard-hit states of the upper Midwest and northern Plains — in Illinois, Wisconsin, Iowa, Minnesota, Montana, Wyoming and the Dakotas — coronavirus case counts have been falling for weeks.
When case counts were skyrocketing across the Upper Midwest earlier this fall, the national media was all over the story. But the near simultaneous regional turnaround that began in mid-November has attracted a lot less attention.

But now, two weeks later — long enough for holiday transmission to start to register in official tallies — there has been no regional post-Thanksgiving spike to speak of.

But the big picture is that since Thanksgiving, the average number of new daily cases has fallen by 19 percent in Illinois; 22 percent in South Dakota; 23 percent in Minnesota; 25 percent in Wisconsin; 27 percent in Montana; 38 percent in Wyoming; 41 percent in Iowa; and 48 percent in North Dakota

Given variations in exposure and individual preventative measures during a pandemic, some experts think spread of the coronavirus could drop drastically once between 35 and 45 percent of a given population has been infected; some have even theorized that a threshold as low as 20 percent could have a similar effect under the right conditions.

This isn’t the same thing as the much-debated herd immunity, which by most calculations requires that about 70 percent of a given population be inoculated (by infection or vaccination) to stop the spread of an epidemic. But the steep and simultaneous decline in cases across the region raises an intriguing question: Is it possible that the virus rapidly infected a large percentage of the low-hanging fruit there — the many people who refused to take precautions, along with those whose working or living conditions made it especially tough to protect themselves — and is now having a harder time reaching the rest of the population?