Nearly 60% of people in the United States, including 3 in 4 children, have now been infected with Omicron or another coronavirus variant, data released Tuesday show.
The new findings, which go through February 2022, highlight just how widely the Omicron variant of the SARS-CoV-2 virus spread in the country. On Tuesday, the virus even reached into the White House, with Vice President Kamala Harris reporting that she had tested positive. She has shown no symptoms, a spokesperson said, and would work outside the White House, at her official residence, until she tested negative.
Before the Omicron variant took off in the United States in December, the portion of the population that had been infected was about 1 in 3, the Centers for Disease Control and Prevention reported.
The updated figures come from a study that has been measuring the so-called seroprevalence of the coronavirus at various points throughout the pandemic. The study relies on testing blood samples from participants for particular antibodies that are generated only by an infection; they are different from the antibodies that Covid-19 vaccines elicit. This is the first time that the population seroprevalence is over 50%.
“We do believe there is a lot of protection in the community both from vaccination as well as from boosting and from prior infection,” CDC Director Rochelle Walensky said in a call with reporters. “That said, we cannot underscore enough, those who have detectable antibodies from infection, we still encourage them to get vaccinated.”
Walensky said cases and hospitalizations were trending upward in upstate New York and the Northeast. But hospitalizations haven’t increased as much as in previous waves, she said, and the need for oxygen or ICU admission has also been lower, all of which she attributed to a larger amount or protection in the larger community, both from disease and from vaccination.
In counties that do have high levels of Covid-19, she said, the CDC recommends people wear masks in public indoor settings to avoid infection and to spare the health system from further strain. Asked about masks on public transportation, she repeated the CDC’s recommendation that people still wear masks on planes, trains, buses, and subways, and the agency’s “disappointment” at a Florida judge’s ruling last week that overturned a national mandate to wear them.
These “sero-surveys” can help provide better estimates for the percentage of the population that has contracted the virus. Official infection numbers are always going to be an undercount, for several reasons. Some people don’t have any symptoms or have such mild cases they never seek testing; these types of infections only became more common as people got vaccinated. Increasingly, people have relied on at-home tests, which do not get tallied in official counts. Some people also do not have easy access to testing.
“We know that the reported cases are just the tip of the iceberg,” Kristie Clarke, co-lead for CDC’s Covid-19 Epidemiology & Surveillance Taskforce Seroprevalence Team, said on the press call. Research to be published soon estimates there are three infections for every reported case, especially during periods like the Omicron surge.
The researchers found vast differences in cumulative infection rates by age group, with rates decreasing among older groups. From December 2021 to February 2022, the seroprevalence among children 11 and younger increased from 44.2% to 75.2%. It grew from 45.6% to 74.2% among those ages 12 to 17.
Among adults 18 to 49, seroprevalence increased from 36.5% to 63.7% over the time period, and from 28.8% to 49.8% among people 50 to 64. Among people 65 and older, seroprevalence rose from 19.1% to 33.2%.
The varying rates reflect how much contact people in different age groups have with others, as well as the continued precautions older people are taking against Covid-19.
The data do not include seroprevalence by race and ethnicity.
For children too young to be vaccinated, “the best way to protect them is to make sure that they are surrounded by people who are taking preventive measures, like staying up to date with their vaccines,” Clarke said. And for children who have had Covid and are 5 or older, “as a pediatrician and a parent, I would absolutely continue to endorse that the children get vaccinated even if they have been previously infected.”
The research team cautioned that the updated findings might in fact also underestimate the total number of SARS-2 infections, in part because infections that happen after vaccination — what are called breakthrough infections — might generate low levels of the antibodies the tests look for.
The high level of population immunity in the United States should help the country better weather future infection waves. While people who’ve been infected or vaccinated can get Covid-19 — particularly if they are months out from their last case or shot, and as the virus keeps evolving — they probably still have protection against severe outcomes.
CDC officials emphasized that compared to infection, vaccination provides a much safer way to build protection against Covid-19. They recommend that even people who have been infected get vaccinated.
Clarke also warned people not to rely on immunity after infection.
“Having infection-induced antibodies does not necessarily mean you are protected against future infection,” she said. “We still do not know how long infection-induced immunity will last, and we cannot know from the study again whether all the people who tested positive for SARS-CoV-2 antibodies continue to have protection from their prior infection.”
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