Epidemiologists say the true number of infections is probably much higher than the official tallies. Even with much more widespread testing now than in the pandemic’s early months, they say, many people who have never experienced symptoms may not have been tested or counted.
Ira Longini, a professor of biostatistics at the University of Florida, estimates that about 20 percent of Americans have had the virus — more than twice the number that is reported. Statistical modeling that he recently completed for Florida suggests that one-third of the state’s population has been infected at some point, quadruple the reported share.
It would take a coordinated nationwide serology study program to move beyond modeling estimates and have a solid grasp of how many people have actually had the virus, he said. The C.D.C. conducts some serology testing, he said, but not enough to provide a full picture.
“The bottom line is, we don’t know, but we can guess from modeling,” Dr. Longini said.
The proportion can vary widely from place to place. In Dewey County, S.D., almost one in four residents has tested positive, but in San Juan County, Wash., only one in 200 has.
Many of the American metropolitan areas with the most reported cases relative to their populations are in the South or Southwest, where the virus has been spreading fast lately, but some are in areas like the Great Plains that were worse off in the fall. The top five are Yuma, Ariz.; Gallup, N.M.; Bismarck, N.D.; and Lubbock and Eagle Pass, Texas.
The metro areas with the greatest number of new cases per capita in the past two weeks reflect the same trend, and also note the virulence of the outbreak in California. Those areas are Laredo and Eagle Pass, Texas; Inland Empire, Calif.; Jefferson, Ga.; and Oxnard, Calif.
More than a million people are known to have tested positive in Los Angeles County, one of the nation’s hot spots over the past few months. And George Rutherford, a professor of epidemiology and biostatistics at the University of California, San Francisco, estimated that the true number of infections there is double that figure, or one out of every five Angelenos.
“It’s not enough for herd immunity, but it’s enough to blunt the curve,” he said.