South Africa recorded its first case of most transmissible COVID-19 variant known as the XBB.1.5 variant

South Africa recorded its first case of most transmissible COVID-19 variant known as the XBB.1.5 variant.

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.”

One thing Dr. Lemieux and other experts are confident about is that XBB.1.5 is not the last chapter in the coronavirus’s evolution. In fact, they expect that a descendant of XBB.1.5 may soon gain mutations that make it even better at spreading.

That descendant may already exist, infecting people without raising notice yet. But sequencing efforts have declined so much worldwide that the discovery of the next generation of XBB.1.5 may be delayed. “As sequencing becomes less and less available at a global level, it’s difficult for us to track each of the subvariants of Omicron,” Dr. Van Kerkhove said.

Scientists have reconstructed the evolution of XBB.1.5 (which some have nicknamed Kraken) by poring over new sequences of coronaviruses in online databases. The first major step came last year when two earlier forms of Omicron infected the same person. As the viruses replicated, their genetic material was shuffled together. A new hybrid form emerged, with genetic material from both viral parents. Virus-watchers named it XBB.

The good news is the worst appears to be over from the RSV surge that has been making life miserable for many children and their parents. RSV cases have been falling steadily since the end of November, according to the Centers for Disease Control and Prevention.

At the same time, the flu — which also came roaring back this fall after mostly disappearing for the previous two years — looks like it's finally receding in most places, according to the latest data out Friday from the CDC.

The rate at which the coronavirus is being detected in wastewater, which has become a bellwether for the pandemic, has tripled or quadrupled in many parts of the U.S. in recent weeks, Jha says. COVID-19 hospitalizations have jumped 70%, he says. And 300 to 400 people are dying every day from COVID-19.

To make matters worse, all this is happening as yet another new, even more transmissible variant has taken over in the United States. Called XBB.1.5, this new omicron subvariant was barely on the radar in late November. But according to new estimates released Friday by the CDC, XBB.1.5 now accounts for almost a third of new infections and is the dominant variant in the Northeast.