60% of Americans have already had COVID-19

 https://scitechdaily.com/1-in-3-americans-already-had-covid-19-by-the-end-of-2020/

A new study published in the journal Nature estimates that 103 million Americans, or 31 percent of the U.S. population, had been infected with SARS-CoV-2 by the end of 2020. Columbia University Mailman School of Public Health researchers modeled the spread of the coronavirus, finding that fewer than one-quarter of infections (22%) were accounted for in cases confirmed through public health reports based on testing.

The study is the first to comprehensively quantify the overall burden and characteristics of COVID-19 in the U.S. during 2020. The researchers simulated the transmission of SARS-CoV-2 within and between all 3,142 U.S. counties using population, mobility, and confirmed case data.

Infections were more widespread in some areas of the country. In areas of the upper Midwest and Mississippi valley, including the Dakotas, Minnesota, Wisconsin, and Iowa, more than 60 percent of the population is estimated to have been infected by the end of 2020. In five metropolitan areas the researchers examined, 48 percent of residents of Chicago, 52 percent of Los Angeles, 42 percent of Miami, 44 percent of New York City, and 27 percent of people in Phoenix, had been infected in the same timeframe.

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

The percentage of people with infections who died from COVID-19 fell from 0.8 percent during the spring wave to 0.3 percent by year’s end. Urban areas like New York City that peaked in the spring saw the worst numbers for reasons that include delays in testing availability and masking mandates, overwhelmed hospitals, and lack of effective treatments.

A new pandemic landscape for 2021. Looking ahead, the authors write that several factors will alter population susceptibility to infection. The virus will continue to spread to those who haven’t yet been infected. While vaccines protect against severe and fatal disease, breakthrough infections, including those that are mild or asymptomatic, will contribute to the spread of the virus. The current study does account for the possibility of reinfection, although there is evidence of waning antibodies and reinfection. New more contagious variants make reinfection and breakthrough infections more likely. 

In the US, there were roughly 20 million confirmed COVID-19 cases in 2020 (16% of population). However, according to this study, 31% of the population was actually infected by the virus. Through 8 months of 2021, there have been about 18 million additional cases. Just taking the 2020 data and applying it to 2021 would add another 30% of actual infections. Notably, testing has improved from early in the pandemic, likely catching more cases now. But, vaccinations have also increased the number of asymptomatic, undetected cases, too. Therefore, let's assume those figures cancel each other out, leaving 60% of the US population having already had COVID by the end of August 2021.