The best- and worst-case scenarios for Covid-19 this winter

The summer wave of Covid-19 cases and deaths is finally starting to fade, but already, winter is within sight.

Last December and January, the United States and much of the world experienced the deadliest surges of the pandemic. At one point, more than 3,000 Americans were dying every day of Covid-19. Nearly 250,000 people died in the United States over December, January, and February.

But this winter may be different, at least in America. Last year, almost nobody was vaccinated against Covid-19. As of October 7, 56 percent of the US population is fully vaccinated,...