Epidemiologists say the actual number of infections is likely much higher than official figures. Even with testing much more widespread now than in the early months of the pandemic, they say, many people who have never had symptoms may not have been tested or counted.
Ira Longini, professor of biostatistics at the University of Florida, estimates that about 20% of Americans have had the virus – more than double the number reported. Statistical modeling he recently completed for Florida suggests that a third of the state’s population has been infected at some point, which is four times the reported share.
There should be national coordination study to go beyond modeling estimates and get a solid understanding of how many people actually had the virus, he said. The CDC is performing serologic testing, he said, but not enough to provide a full picture.
“In the end, we don’t know, but we can guess from the modeling,” Dr. Longini said.
The proportion can vary considerably from place to place. In Dewey County, SD, nearly one in four residents has tested positive, but in San Juan County, Wash., Only one in 200 has it.
Most of the U.S. metropolitan areas with the most reported cases relative to their populations are in the south or southwest, where the virus has spread rapidly in recent times, but some are in areas like the Grandes Plains which were worse off in the fall. The top five are Yuma, Arizona; Gallup, NM; Bismarck, ND; and Lubbock and Eagle Pass, Texas.
The metropolitan areas with the highest number of new cases per capita over the past two weeks reflect the same trend, and also underscore the virulence of the epidemic in California. These areas are Laredo and Eagle Pass, Texas; Inland Empire, California; Jefferson, Georgia; and Oxnard, California.
More than a million people are known to have tested positive in Los Angeles County, one of the nation’s hot spots in recent months. And George Rutherford, professor of epidemiology and biostatistics at the University of California at San Francisco, estimated that the actual number of infections there is double, or one in five Angelenos.
“It‘s not enough for collective immunity, but it’s enough to blunt the curve,” he said.