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Coronavirus has infected majority of Americans, blood tests indicate

But officials caution that people should not presume they have protection against the virus going forward

Pedestrians this month in Philadelphia, where officials reimposed and then dropped a mask mandate as virus cases rise. The CDC estimates that a subvariant now accounts for most cases in the Northeast. (Michelle Gustafson for The Washington Post)
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Before omicron, one-third of Americans had been infected with the coronavirus, but by the end of February, that rate had climbed to nearly 60 percent, including 3 out of 4 children, according to federal health data released Tuesday.

The data from blood tests offers the first evidence that over half the U.S. population, roughly 190 million people, has been infected at least once since the pandemic began. That is more than double the official case count. Many of those infections are likely to have been asymptomatic or with few symptoms. The virus has killed nearly 1 million Americans and caused disruptions that have driven up death rates from other causes, including cancer and heart disease.

Officials cautioned, however, that the data, drawn from tens of thousands of blood samples from across the country, does not indicate people have protection against the virus going forward, especially against increasingly transmissible variants that may be able to evade antibodies. Previous infections are believed to offer some protection against severe disease for most people, especially when combined with vaccinations. But the natural waning of antibodies and an ever-evolving virus create opportunities for reinfection.

“We continue to recommend that everyone be up to date on their vaccinations, get your primary series and booster, when eligible,” CDC Director Rochelle Walensky said during a media briefing.

“We don’t know how long [ago] that infection was,” she said of antibodies from past infections. “We don’t know whether that protection has waned. We don’t know as much about that level of protection than we do about the protection we get from both vaccines and boosters.”

The BA.2 “stealth” omicron variant is expected to soon become the dominant strain. Here is what you need to know about a possible new wave of infections. (Video: Brian Monroe, John Farrell/The Washington Post)

The CDC report offers confirmation of what experts suspected: Most people in the country have had at least one coronavirus infection. Close to half of those got that infection in recent months when the omicron variant swept the nation. That variant, first identified in southern Africa, had so many mutations it could easily infect people who had antibodies from previous bouts of the virus or whose antibodies were generated by a vaccine.

Antibodies serve as a front-line defense against viral infections, and they wane naturally over time. Meanwhile the virus continues to mutate into new variants and subvariants that can partially evade the immunity provided by vaccines or previous infections.

“Anecdotally, I hear a lot about people who have had it two or three times,” said Jeffrey Shaman, an epidemiologist at Columbia University. “We still don’t know how much more this virus can produce radically immune-evasive variants, as we saw with delta and omicron.”

Shaman said he was not surprised by the large percentage of people with antibodies from previous coronavirus infections. “I think there’s an enormous number of undocumented infections,” he added.

Early in the pandemic, some experts had theorized that the country would reach herd immunity — a point at which so many people had immunity from infections or vaccinations that the spread of the virus would be squelched except for rare outbreaks. But the virus appears to be too slippery for that scenario.

Experts said the trajectory of the pandemic will continue to depend on the threat of future virus variants. And they note there is no sign the virus will stop mutating and spewing forth new variants that can evade immunity to some degree and cause “breakthrough” infections among vaccinated people and reinfections in those who have recovered previously.

“There’s also no known threshold of the population where once we get above X percentage we will completely stop community transmission of SARS-CoV-2,” said Kristie Clarke, the CDC official who wrote the report.

Clarke said the latest data shows that by February, “evidence of previous covid-19 infections substantially increased among every age group, compared especially to December 2021,” just as the omicron variant was beginning to surge.

The CDC study was able to distinguish naturally induced antibodies from those generated from vaccines. That allowed researchers to estimate actual infections, separate from the effect from vaccinations.

The largest increases were in children and teenagers through age 17 — about 75 percent of them had been infected by February, based on blood samples that look at antibodies developed in response to a coronavirus infection but not in response to vaccination. That’s about 58 million children. Clarke said the largest increases took place in those with the lowest levels of vaccination, noting that older adults were more likely to be fully vaccinated.

The blood test data suggests 189 million Americans had the coronavirus by end of February, well over double the 80 million cases shown by The Washington Post’s case tracker, which is based on state data of confirmed infections. Clarke said that’s because the blood tests capture asymptomatic cases and others that were never confirmed by coronavirus tests.

With the omicron surge, officials had expected there would be more infections. “But I didn’t expect the increase to be quite this much,” Clarke added.

The blood samples that are the basis of the study come from CDC contracts with commercial blood testing laboratories to perform an extra test for coronavirus antibodies on routine samples taken from physical examinations, regular office visits and pre-surgical tests. The CDC gets about 70,000 blood tests per month. Patient age and state are included but not the patient’s identity.

The test detects antibodies produced for at least one to two years after infection, Clarke said. It is a good measure of how broadly the U.S. population has been infected over the two-plus years of the pandemic but does not capture the extent to which individuals are currently protected against infection, she said.

CDC officials emphasized the continued importance of vaccinations for children who may have been previously infected.

“I could see that some people would wonder with this high seroprevalence regarding vaccinations,” said Clarke, a pediatrician and mother of a 4-year-old. But vaccination protects against many types of coronavirus-related illnesses in children, she said. Covid-19 can be severe in children, with a rare but serious inflammatory syndrome known as MIS-C “happening 60 to 70 percent of the time in previously healthy kids,” she said. Post-covid conditions, or long covid, can also occur.

When new variants such as omicron appear, “that is when we start to see more cases of reinfection,” she said. “So when you’re looking at whether to vaccinate a child, you need to also think about giving them broad protection … not just [from] infection they might have had with omicron. The vaccine … really maximizes the protection to kids.”

The CDC also unveiled updated information on variants Tuesday, estimating that 29 percent of new coronavirus cases nationally are caused by BA.2.12.1, an omicron subvariant that first attracted notice from New York state officials earlier this month.

In parts of the country, the subvariant remains relatively rare, but the CDC estimates that it now accounts for a majority of cases in the Northeast. The upward trajectory suggests it is outcompeting BA.2, another omicron subvariant that became dominant over the past two months, replacing the original omicron strain that infected tens of millions of people from December through February.

Walensky said preliminary evidence shows BA.2.12.1 to be about 25 percent more transmissible than BA.2. But she also noted there is no sign yet of hospitalizations ticking upward at a rate commensurate with the subvariant’s spread, which could be due to “a large amount of protection in the community” from previous infections and vaccinations.

“We’re watching this carefully,” Walensky said. “We are not anticipating more severe disease from some of these subvariants, but we are actively studying it.”