According to a new model produced by Columbia University Mailman School of Public Health researchers, around one in three Americans had been infected with SARS-CoV-2 by the end of 2020. Simulating viral transmission across the whole country, the research suggests the number of officially confirmed cases can only account for a small volume of total infections.
“The vast majority of infectious were not accounted for by the number of confirmed cases,” explains Jeffrey Shaman, one of the researchers working on the study. “It is these undocumented cases, which are often mild or asymptomatic infectious, that allow the virus to spread quickly through the broader population.”
The research collected confirmed COVID-19 case data from 3,142 counties in the US and simulated transmission of the coronavirus across the country in 2020. The model estimated 103 million Americans had been infected with SARS-CoV-2 by December 31, 2020. This accounts for about 31 percent of the total US population.
Overall, confirmed cases only accounted for 22 percent of the total number of infections. This number is almost exactly the same as ongoing estimates from the Centers for Disease Control and Prevention (CDC). The CDC’s surveillance estimates less than a quarter of all SARS-CoV-2 infections in the United States are reported.
Infection rates across the US varied wildly depending on the time of year. Los Angeles County, for example, was estimated to harbor stunningly high infection rates at the very end of 2020, with 2.42 percent of its total population potentially contagious with SARS-CoV-2 on December 31st, whereas Miami experienced its infection rate peak in July when it was estimated 1.25 percent of the total population were infected.
Tracking fatality rates across 2020 the new research calculated a significant drop in COVID-19 mortality from spring to winter. During the pandemic’s initial wave the model estimated 0.8 percent of people died from COVID-19. However, by the end of the year this rate had fallen to just 0.3 percent. The drop in fatality rate is thought to be due to improvements in public health measures, such as mask mandates and better testing, and growing knowledge of how best to treat this new disease.
Despite the significantly high population infection rates in 2020, the researchers indicate 2021 presents an entirely different landscape for the pandemic. Of course, there are still millions of Americans yet to directly encounter the virus and susceptible to infection, and vaccinations will help prevent severe COVID-19 and death despite growing numbers of mild breakthrough infections, but a trio of fundamental factors is set to shape the pandemic over the course of the rest of this year, according to the new study.
“… our model does not represent re-infection, either through waning immunity or immune escape; however, re-infection has been documented, evidence of waning antibody levels exists, and new variants of concern have emerged and will likely continue to do so,” the researchers conclude in the newly published study. “All these processes will affect population susceptibility over time and help determine when society enters a post-pandemic phase.”
The new study was published in the journal Nature.
