“People with mild symptoms, I just send them home,” said Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, and a critical care physician in Pittsburgh. “All of this is creating disparities in case-fatality ratios that don’t reflect real differences.”

Even with testing now becoming more widely available for Americans with Covid-19 symptoms, the proportion of people infected by the virus who do not feel appreciably sick is unknown, including in places with some of the highest deaths per 100,000 people: New York (55), Spain (40), Belgium (36), Italy (35), New Jersey (32), France (23) and Louisiana (22).

A clue comes from Iceland, which has tested 6 percent of its population, perhaps the highest proportion of any country. Of those who tested positive, 43 percent had no symptoms at the time, though it is likely that many developed them later.

What scientists call the infection fatality rate is so closely watched because even a seemingly trivial decrease — from, say, 1.0 percent to 0.9 percent — could mean a few hundred thousand fewer deaths in a population the size of the United States. It is also used to calibrate interventions aimed at preventing more deaths with their grim economic consequences.