Undocumented infections accounted for estimated three-quarters of infections last year
Columbia University's Mailman School of Public Health
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.
Testing picked up on a growing number of infections
but offered an incomplete picture
The portion of
confirmed cases reflected in the study's estimates, i.e. the ascertainment
rate, rose from 11 percent in March to 25 percent in December, reflecting
improved testing capacity, a relaxation of initial restrictions on test usage,
and increasing recognition, concern, and care-seeking among the public.
However, the ascertainment rate remained well below 100 percent, as individuals
with mild or asymptomatic infections, who could still spread the virus, were
less likely to be tested.
"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."
One in 130 Americans was contagious at year's end
Roughly 1 in 130
Americans (0.77%) was contagious with SARS-CoV-2 on December 31, 2020. A
similar percentage (0.83%) was estimated to be latently infected, i.e. infected
but not yet contagious. In some metropolitan areas, the percentage of
individuals who was contagious at year's end was much higher.
Fatality rates fell 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 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.
Cities peaked at different times of the year
New York and
Chicago experienced strong spring and fall/winter waves but little activity
during summer; Los Angeles and Phoenix underwent summer and fall/winter waves;
and Miami experienced all three waves. Los Angeles County, the largest county
in the U.S. with a population of more than 10 million people, was particularly
hard-hit during the fall and winter and had a community infection rate of 2.4
percent on December 31.
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.
"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," concludes Sen Pei, PhD, assistant professor of
environmental health sciences at Columbia University Mailman School of Public
Health.
Additional
authors include Teresa K. Yamana, Sasikiran Kandula, and Marta Galanti at
Columbia Mailman School.
The study was
supported by funding from the National Science Foundation (DMS-2027369) and a
gift from the Morris-Singer Foundation. Human mobility data was provided by
SafeGraph. Shaman and Columbia University disclose partial ownership of SK
Analytics. Shaman discloses consulting for Business Network International
(BNI).
