Amid worries about Omicron, virus cases are jumping across the United States.

Offices are canceling holiday parties. Broadway performances are being shuttered because of breakthrough infections among the casts. College finals are being moved online.

Coronavirus cases are spiking again in New York City, past anything seen since the end of last winter’s surge, and in much of the United States, where the Delta variant remains by far the dominant version. Across the country, the average of new cases a day has jumped to more than 120,000 — a 40 percent increase from two weeks ago, according to a New York Times database, and 70 percent more than when...