Nearly one-third of the entire U.S. population had been infected with COVID-19 by the end of 2020 — a much higher rate of infection than previously known, a new study has found.
The study, which was conducted by researchers at Columbia University’s Mailman School of Public Health and published in the journal Nature on Thursday, reveals that 103 million Americans, or 31% of the population, had been infected by December 2020 — and that the rate of infection in New York City at the time was even higher at 44%.
“The vast majority of infectious were not accounted for by the number of confirmed cases,” said Jeffrey Shaman, one of the report’s researchers and a professor of environmental health sciences at the Mailman School. “It is these undocumented cases, which are often mild or asymptomatic infectious, that allow the virus to spread quickly through the broader population.”
The study aimed to account for a lack of testing early in the pandemic and the fact that people with no or few symptoms were less likely to get tested.
Its findings, when paired with COVID-19 test results from the end of December 2020, reveal a stark divide.
While the study estimates that 31% of the overall U.S. population had been infected by December, the Centers for Disease Control put out statistics that same month showing only 12.3% of COVID-19 tests taken at the time were positive.
The peer-reviewed research found that by the end of 2020, infection rates were particularly high in several regions and in big cities, with 48% of people in Chicago infected, 52% of Los Angeles and 42% of Miami.
In parts of the Midwest like Minnesota, Wisconsin and Iowa, more than 60% of the population had been infected, the study found.
“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, one of the researchers and an assistant professor at Mailman.