Undocumented infections accounted for estimated three-quarters of infections last year
From: Columbia University Public Health
Now
August 26, 2021 – 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.
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