A new study estimates that nearly one-third of the U.S. population — 103 million Americans — may have contracted COVID-19 by the end of 2020, with only a fraction of those cases correctly reported in public health reports.
Columbia University Mailman School of Public Health researchers found that those with mild to no symptoms were not likely to report their infections and exacerbated the spread of the virus in a new study, published Thursday in Nature magazine.
“The vast majority of infections were not accounted for by the number of confirmed cases,” Jeffrey Shaman, professor of environmental health sciences at Columbia University Mailman School of Public Health said in a press release. “It is these undocumented cases, which are often mild or asymptomatic infectious, that allow the virus to spread quickly through the broader population.”
The rate at which likely cases were confirmed — the “ascertainment rate” — rose from 11 percent to 25 percent from March to December as testing availability and accuracy increased.
Concurrently, the death rate dropped from 0.8 percent to 0.3 percent.
The study found certain parts of the country experienced heightened infection rates: Over 60 percent of the population of the Upper Midwest and Mississippi Valley including North and South Dakota, Minnesota, Wisconsin and Iowa was infected by the year’s end, it found.
Researchers analyzed five major metropolitan areas, in which it found that 52 percent of Los Angeles residents, 48 percent of Chicago, 44 percent of New York City, 44 percent of Miami and 27 percent of Phoenix had contracted the virus by the end of 2020.
The study found seasonal surges in the virus in each city, with strong surges in the spring and fall/winter in New York and Chicago while calming down in the summer. Alternatively, L.A. and Phoenix underwent summer and fall/winter waves. Miami experienced all three, the student 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, PhD, assistant professor of environmental health sciences at Columbia University Mailman School of Public Health, in the release.
Hospitalizations this week reached levels not seen since January, with over 100,000 COVID-19 hospitalizations reported on Thursday.
The surge comes amid the fourth wave of the virus and its highly contagious Delta variant, which killed 1,456 Americans Wednesday, according to Johns Hopkins data. The daily record high of 4,460 was set on Jan. 12.