OVERVIEW: CDC changes guidlines to exclude persons exposed to the virus but don't show symptoms.

The C.D.C. changes testing guidelines to exclude those exposed to virus who don’t exhibit symptoms.

The Centers for Disease Control and Prevention quietly modified its coronavirus testing guidelines this week to exclude people who do not have symptoms of Covid-19 — even if they have been recently exposed to the virus.

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

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

A more lax approach to testing, experts said, could delay crucial treatments, as well as obscure the coronavirus’s true spread in the community. Case numbers remain persistently high across much of the United States, though they have been falling in recent weeks, to an average of about 43,000 new cases a day from a peak of more than 66,000 a month ago. Many of the states that saw the largest outbreaks in early summer are now reporting sustained progress, including Arizona and Florida. But parts of the Midwest, as well as Hawaii and some U.S. territories, are still seeing increases in new cases. ...