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

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

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

Although there is no evidence X BB. 1.5 is more virulent, a recent swirl of theories linking the rise of new variants to vaccination has cast a spotlight on this latest strain and raised concerns among experts that such misinformation could further depress demand for booster shots.

“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 in cases last winter, it has spawned a host of descendants that are m ore adept at slipping past antibodies. 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, but it doesn’t necessarily worsen symptoms.

Health officials are urging Americans to get a booster dose of the new vaccine for omicron subvariants in order to protect against an expected winter surge in cases from holiday gatherings.

The nation was 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 of 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 — 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,” wrote Ashish Jha , coordinator of the White House’s virus response, in a Jan. 4 tweet. “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.

“ Things like boosters are always beneficial,” said Kristian Andersen , a professor o f immunology 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 drawing criticism. An opinion piece published in the Wall Street Journal by a member of its editorial board suggested that repeated vaccinations 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 evolution of the virus, so there is no reason to blame shots for the variants.

But he said it’s difficult to justify regular booster shots when the protection against mild illness is brief and immunity from earlier shots or infections still provides robust protection.

“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, wrote noted there is no clear evidence to support the thesis of the Wall Street Journal piece.