“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.

Temperature checks are akin to “getting the oil checked before you go on a long car trip,” said Dr. David Thomas, an infectious disease specialist at Johns Hopkins University School of Medicine. “It makes you feel better, but it’s not going to keep you from wrecking the car or prevent the tires from falling off. It’s not going to make your trip any safer.”

“It’s something you can do, and it makes you feel like you’re doing something,” he said. “But it won’t catch most people who are spreading Covid.”

Most people who spike a fever feel lousy, and presumably would cancel their dinner plans, said Dr. Thomas McGinn, Northwell Health’s senior vice president and deputy physician in chief. Temperature checks might pick up a few individuals who are unaware of their fever, he said.

But the absence of fever “means nothing,” he said. “It’s not a very sensitive test.”

It does, however, convey a strong public health message, serving as a reminder that people must take precautions, and that itself may be of benefit, Dr. McGinn said. “It makes people think twice, and reminds them that this is a big deal, we still need to be careful, you need someone to stand by the door to do that,” he said.

But here’s the rub: While fever can be a symptom of Covid-19, the disease caused by the coronavirus, not everyone infected with the virus develops a fever, or many other symptoms, for that matter. Physicians writing in the New England Journal of Medicine have called the phenomenon of symptomless spread the “Achilles’ heel of Covid-19 pandemic control.”

Evidence of asymptomatic spread dates back to early in the pandemic, but has been mounting ever since. A recent study from South Korea published in JAMA Internal Medicine in August offered even more proof, finding that infected individuals who don’t feel ill may carry just as much virus in their nose, throat and lungs as those with symptoms — and for almost as long.



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