By the end of 2020, 103 million Americans (31 percent), or about a third of the US population, had been infected with SARS-CoV-2. This is the conclusion reached by scientists at the University of Columbia. This means that less than a quarter of infections (22 percent) are cases confirmed in public health reports based on COVID-19 testing. This is reported in an article published in the journal Nature.
Researchers have modeled SARS-CoV-2 transmission across and between 3142 US counties using population, mobility, and confirmed case data. In some parts of the country, the infection has become more widespread. Scientists estimate that by the end of 2020, more than 60 percent of the population in the Upper Midwest and Mississippi Valley, including the states of Dakota, Minnesota, Wisconsin and Iowa, were infected. Also infected were 48 percent of Chicagoans, 52 percent of Los Angeles, 42 percent of Miami, 44 percent of New Yorkers and 27 percent of Phoenix residents.
Testing uncovered a growing number of infections, but did not give the true picture. The proportion of confirmed cases, reflected in researchers' estimates, rose from 11 percent in March to 25 percent in December, reflecting improved testing capabilities. However, the detection rate remained well below 100 percent. The vast majority of infectious diseases are not attributable to the number of confirmed cases.
The death rate has declined thanks to increased treatment and public health measures. The proportion of those infected who died from COVID-19 fell from 0.8 percent during the spring surge to 0.3 percent by the end of the year.
