Now the contagious Omicron variant is pushing the C.D.C. into uncharted territory. Because decisions must be made at a breakneck pace, the agency has issued recommendations based on what once would have been considered insufficient evidence, amid growing public concern about how these guidelines affect the economy and education.

The agency’s director, Dr. Rochelle P. Walensky, has sometimes skipped much of the traditional scientific review process, most recently in shortening the isolation period for infected Americans…

In recent interviews, some officials at the C.D.C. privately described the decisions as demoralizing, and worried about Dr. Walensky’s increasing reliance on a small group of advisers and what they saw as the White House’s heavy political influence on her actions.

Yet others outside the agency commended Dr. Walensky for short-circuiting a laborious process and taking a pragmatic approach to managing a national emergency, saying she was right to move ahead even when the data was unclear and agency researchers remained unsure.