“We know that increased vaccination and infection are strengthening our defenses against Covid,” Thomas Frieden, a former chief of the U.S. Centers for Disease Control and Prevention, wrote Monday on Twitter. “I’m more optimistic about our ability to tame the pandemic than at any point since its emergence.”

“Unless, of course, a worse variant emerges, with the infectivity of Omicron and as deadly as Delta,” he added.

In the United States, Omicron cases appear to have crested in the Northeast, parts of the Upper Midwest and other areas where it first arrived, while nationally, new cases and hospital admissions have leveled off in recent days.

Still, hospitals in other areas across the country remain overstretched, straining to handle patients after multiple surges and staffing shortages, including in Mississippi, where nearly all of the state’s acute-care hospitals have been pushed to capacity. And new deaths remain high.

In the United States, 37 percent of people are not fully vaccinated, compared with 25 percent in Western Europe. Three-quarters of the U.S. population has not had a booster shot, versus half of Western Europeans.

Devi Sridhar, the head of the global public health program at the University of Edinburgh in Scotland, said that the number and concentration of unvaccinated people in parts of the United States put the country in a more dangerous position than well-vaccinated parts of Europe, where the return of normalcy was underway.

“In these countries, we’re getting to what I think of as the beginning of the end, and we have the tools to manage it,” she said. “We have to shift to the next chapter of this pandemic, and move from an emergency crisis to one which is more sustainable.”