A recent CDC study found that around 140 million Americans have been infected by the coronavirus, almost double the number regularly reported in national Covid-19 case counts. And while some experts say this increase in population immunity could help stem future surges, others remain cautious about potential new variants, as well as waning immunity in the future.
For the study, CDC analyzed 72,000 blood samples collected by commercial labs nationwide between Dec. 27, 2021, and Jan. 29, 2022. The blood samples were originally sent to labs for reasons unrelated to Covid-19 but were also tested for coronavirus antibodies. The study counted each person only once and included only antibodies from natural infection, including asymptomatic cases.
Overall, the study found that 43.3% of the total U.S. population, or around 140 million people, have coronavirus antibodies. This is almost double the approximately 74 million Covid-19 cases reported nationwide as of Jan. 31.
A previous update to the study at the end of November estimated that 103 million Americans had been infected. This means around 37 million people contracted Covid-19 in the two months ending in late January—the start of the omicron wave.
Among all 50 states, cumulative infection rates ranged from 18% in Vermont to 56% in Wisconsin. In 14 states, including Georgia, Illinois, Texas, and New Jersey, at least half of the population has had Covid-19.
In addition, the study found that over 58% of children under 18 have been infected over the course of the pandemic. Adults have had lower infection rates, with 48% of those 18 to 49 years old, 37% of those 50 to 64 years old, and 23% of those 65 years and older having been infected.
Several infectious disease experts have said omicron's rapid spread would likely help increase overall population immunity in the United States.
According to Virginia Pitzer, an epidemiologist at the Yale School of Public Health, around 90% to 95% of all Americans will have some form of immunity against the coronavirus after the current omicron surge subsides.
"Few people will be naïve—completely naïve, no protection from either vaccination or natural infection—when the omicron wave is over," said Cecile Viboud, an infectious diseases epidemiologist and modeler at NIH's Fogarty International Center.
Separately, Scott Hensley, a vaccine researcher at the Institute for Immunology at the University of Pennsylvania, said that a decrease in viral transmission after the omicron wave could reduce the risk of new variants. "None of us think the virus is going to go away, but the virus will have less opportunity to change because there will be fewer hosts that it can replicate in," he said. "And in an immune population, due to immunity, disease severity will be less."
However, other experts noted warned that there is still potential for new, more dangerous variants to evolve.
According to Deepta Bhattacharya, an immunologist at the University of Arizona, omicron is "not a derivative of delta and so that's what makes it a little unpredictable as to what [variant is] going to come next."
In addition, protection against infection from both virus- and vaccine-induced immunity wanes over time, which means many people who are well-protected now will be vulnerable again in a few months. (Protection against severe disease, on the other hand, appears so far to be more robust.)
Different areas of the country will also not have the same level of immunity, either from natural infection or vaccine-induced immunity. There are still plenty of "pockets that may have not yet had exposure to vaccination or the virus," said Bertha Hidalgo, an epidemiologist at the University of Alabama at Birmingham. This means that areas with comparatively less immunity will be hit harder by any future outbreaks of the coronavirus.
"Some people will be left with immune houses of straw, others of wood, others of brick," said Anne Sosin, a health equity researcher at Dartmouth University.
Overall, William Powderly, head of the Institute for Public Health at Washington University in St. Louis, said scientists have to acknowledge "the necessary humility" that they don't know what will happen next in the pandemic, particularly due to the variability in the coronavirus. (Keating, Washington Post, 2/28; Masson, Becker's Hospital Review, 3/1; Choi, The Hill, 3/1; CDC Covid-19 seroprevalence tracker, accessed 3/2)
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