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Three years after the novel coronavirus emerged, a new variant, XBB.1.5, is quickly becoming the dominant strain in parts of the United States because of a potent mix of mutations that makes it easier to spread broadly, including among those who have been previously infected or vaccinated.

XBB.1.5, pegged by the World Health Organization as “the most transmissible” descendant yet of the omicron variant, rose from barely 2% of U.S. cases at the start of December to more than 27% the first week of January, according to new estimates by the Centers for Disease Control and Prevention.

More than 70% of cases in the Northeast are believed to be XBB.1.5.

While there is no evidence so far that XBB.1.5 is more virulent than its predecessors, a recent swirl of misinformation linking the rise of new variants to vaccination has cast a spotlight on this latest strain and raised concern among some health experts that it could further limit booster uptake.

“XBB did not evolve because people were vaccinated,” said Vaughn Cooper, a professor of evolutionary biology at the University of Pittsburgh. “The way it evolved, let’s be straight, is because people were infected by multiple viruses at the same time.”

Since the omicron variant ignited an explosion of cases last winter, it spawned a host of descendants that are even more adept at slipping past antibodies and caused most infections in the United States. The XBB line emerged as a result of two other omicron subvariants swapping parts.

Virologists who studied XBB.1.5 say it does not appear better at escaping antibodies than other immune-evading predecessors, but it is better at binding to the cell and replicating. That means it can more easily become the dominant strain in a community than its relatives, but it doesn’t necessarily worsen an individual’s COVID symptoms.

Health officials are urging Americans to get a booster dose of the new coronavirus vaccine formulated for omicron subvariants to protect against an expected winter surge in cases from holiday gatherings and as people continue to congregate indoors.

The nation is averaging nearly 48,000 people hospitalized with COVID-19 over a seven-day period, the highest since late February but far below the 150,000 peak set about a year ago. The average daily count of hospitalized patients is up by about 20,000 since Thanksgiving.

But only 15% of those eligible — and 38% of senior citizens, who are most at risk - have received the updated shots.

“For folks without a very recent infection or a bivalent vaccine, you likely have very little protection against infection. And for older folks, diminishing protection against serious illness,” Ashish Jha, coordinator of the White House’s COVID-19 response, tweeted Jan. 4. “So am I concerned about XBB.1.5? Yes. Am I worried this represents some huge set back? No.”

XBB prompted some concerns because of lab studies showing antibodies generated by vaccines struggle to block that variant. But that doesn’t mean the vaccines don’t work: For much of the last year, waves of infections from immune-evading variants have not caused surges of severe illness at hospitals, and experts believe that widespread immunity from shots and prior infections are making people more resilient to the virus.

While antibodies do not provide foolproof shields against reinfections, they still reduce the odds of contracting the virus and greatly reduce the likelihood of severe illness and hospitalization.

“Things like boosters are always beneficial,” said Kristian Andersen, a professor in the department of immunology and microbiology who tracks coronavirus variants at the Scripps Research Institute. “Even if you get infected, you are expected to have less viral load, and you are expected to be able to transmit the virus less.”

But the focus on boosters is also drawing criticism. An opinion piece published in the Wall Street Journal by a member of its editorial board suggested that repeated vaccination made Americans more susceptible to new variants.

Paul Offit, director of the vaccine education center at the Children’s Hospital of Philadelphia, said the risk of mild breakthrough infection has been consistent throughout the pandemic despite the virus’s evolution, so there is no reason to blame shots for the variants. But he said it’s difficult to justify regular booster shots for the general population when the protection against mild illness is brief and immunity from earlier shots or infections still provides robust protection against severe outcomes.

“At some level, we are going to have to get used to mild illness unless you want to take three doses a year and stay in your house most of the time,” said Offit, who also advises the Food and Drug Administration on vaccines.

FDA commissioner Robert Califf, in a series of tweets he said he wrote to address prevalent “misinformation/disinformation,” noted that there was no clear evidence to support the thesis of the Wall Street Journal piece.

“Raising doubts about the value of vaccines will continue to lead well-intended people to delay getting an updated vaccine, leading to a clearly documented risk of death or severe illness,” Califf tweeted, after Sen. Ron Johnson (R-Wis.); Richard Urso, an ophthalmologist and member of America’s Frontline Doctors, a group that spreads COVID misinformation; and many others cited the Wall Street Journal piece as they questioned the use of coronavirus vaccines.

Several studies have tried to examine whether SARS-CoV-2 variants spread faster in locations with higher vaccination rates and found no evidence to support it, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Center in Seattle.

“XBB.1.5 is just sort of the latest variant in what is going to be a continual process of the virus evolving,” Bloom said. “There’s a lot of space for the virus to get mutations.”

While XBB.1.5’s mutations have sparked scientific intrigue, experts caution it doesn’t substantially change circumstances for ordinary people navigating pandemic life with omicron’s descendants as the dominant strains since last spring. In addition to staying up to date on boosters, don’t assume you can’t get sick again if your last shot or bout with COVID was more than several months ago. Wear high-quality masks and avoid crowds to avoid infections that can still cause mild illness, disrupt plans and lead to long-term health complications.

The World Health Organization is monitoring XBB.1.5 because of how quickly it has supplanted other subvariants — and each wave of mass infection brings opportunities for the virus to morph into something more dangerous.

“The more this virus circulates, the more opportunities it will have to change,” said Maria Van Kerkhove, WHO’s technical lead on COVID-19 at a Wednesday news conference. “We do expect further waves of infection around the world but that doesn’t have to translate into further waves of death because our countermeasures continue to work.”

XBB.1.5 surged first in the Northeast, where it now makes up nearly three quarters of cases. But hospitals there have not raised alarms about people coming in sicker because of the new subvariant.

Official case counts are no longer considered reliable because at-home test results are not reported, but wastewater tracking and other measures suggest cases are rising across the country.

At Northwell Health, New York’s largest hospital chain, the ranks of patients testing positive rapidly increased after Thanksgiving, along with the share of viruses confirmed to be the XBB.1.5 strain in lab testing. The “vast majority” of patients with coronavirus are admitted for other causes or coronavirus is a complicating factor, Northwell’s chief of infectious diseases Bruce Farber said. The share of those patients who received intensive care or were placed on ventilators remained stable after the rise of XBB.1.5 in early December. “In terms of very sick people with COVID, it’s almost exclusively elderly and people with a lot of morbidities or who are immunosuppressed,” he said.

Experts hope hospitals in other parts of the country can share Northwell’s experience: weathering a rise of cases without COVID wards packed with people fighting for their lives.

“Even though this thing is continuing to evolve, if you take a step back and look at the big peaks, they are not as ferocious as let’s say delta in the last half of 2021 or the early BA.1 omicron wave,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. “Robust population immunity helps communities withstand new variants.”

The Washington Post’s Carolyn Y. Johnson contributed to this report.

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