By November 2021, nearly two years after the coronavirus emerged in Wuhan and spread across the world, the surprises seemed to be over. More than four billion people had been vaccinated against the virus, and five million had died. Two new variants, known as Alpha and Delta, had surged and then ebbed. As Thanksgiving approached, many Americans were planning to resume traveling for the holiday.

And then, the day after turkey, the pandemic delivered a big new surprise.