Experts questioned the revision, pointing to the importance of identifying infections in the small window immediately before the onset of symptoms, when many individuals appear to be most contagious.

Models suggest that about half of transmission events can be traced back to individuals still in this so-called pre-symptomatic stage, before they start to feel ill — if they ever feel sick at all.

“This is potentially dangerous,” said Dr. Krutika Kuppalli, an infectious disease physician in Palo Alto, Calif. Restricting testing to only people with obvious symptoms of Covid-19 means “you’re not looking for a lot of people who are potential spreaders of disease,” she added. “I feel like this is going to make things worse.”

At a moment when experts have almost universally come forward to encourage more frequent and widespread testing, especially to reach vulnerable and marginalized sectors of the population, the C.D.C.’s update appears counterintuitive and “very strange,” said Susan Butler-Wu, a clinical microbiologist at the Keck School of Medicine of the University of Southern California.