1 in 3 Americans Already Had COVID-19 by the End of 2020

Undocumented infections accounted for an estimated three quarters of infections last year.

A new study published in the journal Nature estimates that by the end of 2020, 103 million Americans, or 31 percent of the US population, were infected with SARS-CoV-2. Researchers from the Columbia University Mailman School of Public Health modeled the spread of the virus. coronavirus, which found that less than a quarter of infections (22%) were identified in cases confirmed by test-based public health reports.

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

In some parts of the country, the infections were more widespread. In areas of the upper Midwest and the Mississippi Valley, including the Dakotas, Minnesota, Wisconsin and Iowa, more than 60 percent of the population is estimated to be infected by the end of 2020. In five metropolitan areas the researchers surveyed, 48 percent of Chicago residents, 52 percent of Los Angeles, 42 percent of Miami, 44 percent of New York City and 27 percent of Phoenix residents had been infected during the same time frame.

Tests picked up a growing number of infections, but provided an incomplete picture. The proportion of confirmed cases reflected in the study’s estimates, i.e. the determination rate, increased from 11 percent in March to 25 percent in December, as a result of improved testing capacity, an easing of initial restrictions on testing and increasing recognition, concern and concern – search among the public. However, the detection rate remained well below 100 percent because individuals with mild or asymptomatic infections, who could still spread the virus, were less likely to be tested.

“The vast majority of infectious diseases were not explained by the number of confirmed cases,” said Jeffrey Shaman, PhD, professor of environmental health sciences at Columbia University Mailman School of Public Health. “It is these undocumented cases, which are often mildly or asymptomatically contagious, which allow the virus to spread rapidly through the wider population.”

One in 130 Americans was contagious by the end of the year. About 1 in 130 Americans (0.77%) was contagious with SARS-CoV-2 on December 31, 2020. A similar percentage (0.83%) were latently infected, i.e. infected but not yet infectious. In some metropolitan areas, the percentage of people who were contagious at the end of the year was much higher.

The death rate declined with strengthening treatments and public health measures. 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 the end of the year. Urban areas like New York City, which peaked in the spring, saw the worst numbers for reasons such as delays in availability testing and masking mandates, overwhelmed hospitals and lack of effective treatments.

Cities peaked at different times of the year. New York and Chicago had strong spring and fall/winter waves, but little activity in the summer; Los Angeles and Phoenix experienced summer and fall/winter waves; and Miami experienced all three waves. Los Angeles County, the largest county in the US with a population of more than 10 million people, was particularly hard hit during the fall and winter and had a 2.4 percent infection rate as of December 31.

A new pandemic landscape for 2021. Looking ahead, the authors write that several factors will change the population’s susceptibility to infections. The virus will continue to spread to those who are not yet infected. While vaccines protect against serious and deadly diseases, breakthrough infections, whether mild or asymptomatic, will contribute to the spread of the virus. The current study does take into account the possibility of reinfection, although there is evidence of declining antibodies and reinfection. New, more contagious variants make reinfection and breakthrough infections more likely.

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

Reference: “Covid-19 Burden and Characteristics in the United States During 2020” by Sen Pei, Teresa K. Yamana, Sasikiran Kandula, Marta Galanti, and Jeffrey Shaman, Aug. 26, 2021, Nature.
DOI: 10.1038/s41586-021-03914-4

Other authors include Teresa K. Yamana, Sasikiran Kandula, and Marta Galanti of the Columbia Mailman School.

The study was supported by funding from the National Science Foundation (DMS-2027369) and a grant from the Morris-Singer Foundation. Human mobility data is provided by SafeGraph. Shaman and Columbia University disclose partial ownership of SK Analytics. Shaman unveils consulting for Business Network International (BNI).