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“Even with the recent increase in Omicron variant, we expect that these prevention strategies will continue to work,” Kristen Nordlund, a C.D.C. spokeswoman, said in a statement on Friday. “However, as we learn more about the Omicron variant, C.D.C. will continue to review and update guidance as needed.”

The new policy, hinted at in the winter Covid-19 plan that President Biden unveiled this month, still calls on students to wear masks and socially distance, and applies only to those who remain asymptomatic. Until now, unvaccinated students were expected to quarantine at home for as long as two weeks after exposure. Some states have had tens of thousands of students in quarantine.

“While over 99 percent of schools are open now, we need to make sure we keep that throughout the winter,” Mr. Biden said as he announced the plan. “We want our children in school.”

Vaccinated students with exposures have generally been allowed to remain in school as long as they are asymptomatic and wear a mask. Dr. Rochelle P. Walensky, the C.D.C. director, said at a news conference on Friday that students participating in test-to-stay programs should be tested at least twice during the seven-day period after an exposure.

In one of the studies the C.D.C. released on Friday, students at schools in Los Angeles County that did not participate in a pilot test-to-stay program, and who had to quarantine, lost an estimated 92,455 in-person school days from Sept. 20 to Oct. 31. In schools participating in the pilot, students exposed to the virus lost no days. Those schools also did not see increases in virus rates among students.