Are we at the beginning of the end of COVID-19? The tricky road to herd immunity, explained

One year into a changed world, the numbers defy comprehension. More than 21 million confirmed cases of COVID-19 in the United States—a rate approaching one in 15 people—plus untold millions who had mild or no symptoms and were never identified. Add the millions who have now received their first doses of vaccine, and it is fair to wonder: