Delta is likely to infect “large numbers” of unvaccinated people, he said.

Will it cause a new surge?

The pandemic is waning in the United States, with cases, hospitalizations and deaths all on the decline. The seven-day case average, roughly 10,350 a day, is the lowest since March 2020, Dr. Rochelle Walensky, director of the C.D.C., said at the briefing on Tuesday. “These numbers demonstrate the extraordinary progress we’ve made against a formidable foe,” she said.

So while Delta may account for an increasing percentage of cases, it is not yet clear whether it will drive the total number of cases higher.

“I think we are not going to see another big, national surge in the United States because we have enough vaccination to prevent that,” Dr. Osterholm said.

Still, vaccination rates have been highly uneven, and are lower in certain states and demographic groups. Delta could fuel outbreaks in the South, where vaccinations lag, or among young people, who are less likely to be vaccinated than their elders.

“In places where there’s still a lot of susceptibility to the virus, it opens a window for cases to start going up again,” said Justin Lessler, an infectious disease epidemiologist at Johns Hopkins University. “But even in those states, and certainly nationally, we’re probably not getting back to the numbers we were seeing last winter.”

Still, he said, it could prolong our path out of the pandemic. “It continues the doldrums,” he said.

What can I do?

Get vaccinated. If you’re already vaccinated, encourage your family, friends and neighbors to get vaccinated. Vaccination is likely to slow the spread of all the variants and reduce the odds that new, even more dangerous variants emerge.