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Model estimates COVID-19 infected nearly a third of Americans in 2020

America reported the highest rate of COVID-19 infections globally last year, but even 20 million was an undercount, according to research published on Thursday estimating 31% of the country contracted the infectious disease in 2020.

“The vast majority of infectious were not accounted for by the number of confirmed cases,” said Jeffrey Shaman, a professor of environmental health sciences at Columbia University Mailman School of Public Health in a statement. “It is these undocumented cases, which are often mild or asymptomatic infectious, that allow the virus to spread quickly through the broader population.”

A majority of Americans avoided infection during 2020, according to the study published in the journal Nature by Columbia University researchers.

To date, the Centers for Disease Control and Prevention tracked 38.3 million cases of COVID-19 and attributed 629,139 deaths to the disease. With a minority of individuals experiencing symptoms and the lack of access to tests early in the pandemic, researchers have long described the official count as an undercount. Models developed by researchers at Colombia University estimate as many as 78% of COVID-19 cases went unreported last year.

Researchers compared model projections with reported infection rates across 3,142 counties pulling from data collected by GitHub, Johns Hopkins University, and the U.S. Census Bureau, among others.

By March 2020, the model estimates the virus infected 11% of the country and 24% of people during the December 2020 surge. Rates of infection varied widely by county during different periods of time.

More than 60% of people in the Dakotas, Minnesota, Wisconsin and Iowa were likely infected by COVID-19 in 2020 according to the model, while only 27% of Phoenix residents were infected.

Researchers additionally found about half of residents in Chicago, Los Angeles and New York City contracted COVID-19 through the first year of the pandemic.

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"The COVID-19 pandemic disrupted health systems and economies throughout the world during 2020 and was particularly devastating for the United States, which experienced the highest numbers of reported cases and deaths during 2020,” the researchers explain in the paper.

Researchers observed three major waves of viral infection beginning with a spring 2020 surge, followed by a in summer cases across southern states and a winter tidal wave.

First submitted for review in February, the paper predicted two main factors would impact population susceptibility to COVID-19 throughout 2021: increasing infections and vaccination shrinking the pool of people who hadn’t been exposed to the disease in the wild.

“While the landscape has changed with the availability of vaccines and the spread of new variants, it is important to recognize just how dangerous the pandemic was in its first year,” said Sen Pei, assistant professor of environmental health sciences at Columbia University Mailman School of Public Health in a statement.

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Models developed by researchers at Columbia University estimate as many as 78% of COVID-19 cases went unreported last year.

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