Health Experts Warily Eye XBB.1.5, the Latest Omicron Subvariant

By Carl Zimmer in the NYT

Three years into the pandemic, the coronavirus continues to impress virologists with its swift evolution.

A young version, known as XBB.1.5, has quickly been spreading in the United States over the past few weeks. As of Friday, the Centers for Disease Control estimated that it made up 72 percent of new cases in the Northeast and 27.6 percent of cases across the country.

The new subvariant, first sampled in the fall in New York State, has a potent array of mutations that appear to help it evade immune defenses and improve its ability to invade cells.

“It is the most transmissible variant that has been detected yet,” Maria Van Kerkhove, the Covid-19 technical lead at the World Health Organization, said at a news conference on Wednesday.

XBB.1.5 remains rare in much of the world. But Tom Wenseleers, an evolutionary biologist at KU Leuven in Belgium, expects it to spread quickly and globally. “We’ll have another infection wave, most likely,” he said.

Advisers at W.H.O. are assessing the risk that XBB.1.5 poses. Jacob Lemieux, an infectious disease doctor at Massachusetts General Hospital, said that the surge in cases would not match the first Omicron spike that Americans experienced a year ago. “Is it a Category Five hurricane?” he said. “No.”

Still, he warned that XBB.1.5 could worsen what is already shaping up to be a rough Covid winter, as people gather indoors and don’t receive boosters that can ward off severe disease.

Dr. Ashish K. Jha, the White House Covid-19 response coordinator, said that the Biden administration was monitoring the emergence of XBB.1.5 and urging people to take advantage of existing countermeasures. Preliminary studies suggest that bivalent vaccines should provide decent protection against XBB and its descendants. Paxlovid will also remain effective at fighting infections.

“We feel pretty comfortable that our countermeasures are going to continue to work,” Dr. Jha said. “But we’ve got to make sure people are using them.”

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