Reversing a decision made just two months ago, the Centers for Disease Control and Prevention is expected to recommend on Tuesday that people vaccinated for the coronavirus resume wearing masks indoors in certain areas of the country.

The change follows reports of rising breakthrough infections with the Delta variant of the virus in people who were fully immunized, as well as surges in cases in regions with low vaccination rates. But the new guidance would mark a sharp turnabout from the agency’s position since May that vaccinated people do not need to wear masks in most indoor spaces.

As recently as last week, an agency spokesman said that the C.D.C. had no plans to change its guidance, unless there were a significant change in the science. Federal officials met on Sunday night to review new evidence that may have prompted the reversal, CNN reported on Tuesday.