While temperature checks may identify people who are seriously ill, those are the people who probably won’t be socializing much or going out for meals. And a growing body of evidence suggests that many of those who are driving transmission are so-called silent carriers — people who have been infected but feel fine, and don’t have a fever or any other symptoms.

Earlier this week, the C.D.C. — which in May told employers to consider checking workers daily for symptoms like fever, but appeared to reverse itself in July — said it would stop requiring airport health screenings beginning Sept. 14 for international passengers from China, Iran, Brazil and other countries because the checks can’t identify silent carriers.

“We now have a better understanding of Covid-19 transmission that indicates symptom-based screening has limited effectiveness because people with Covid-19 may have no symptoms or fever at the time of screening, or only mild symptoms,” the C.D.C. said in a statement.