Three years after the pandemic, the coronavirus continues to impress virologists with its rapid evolution.

A younger version known as XBB.1.5 has been spreading rapidly in the United States over the past few weeks. till FridayThe Centers for Disease Control estimated that it made up 72 percent of new cases in the Northeast and 27.6 percent of cases nationwide.

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

“This is the most transmissible variant found so far,” Maria van Kerkhove, the COVID-19 technical lead at the World Health Organization, told a news conference on Wednesday.

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

WHO advisors are assessing the risk posed by XBB.1.5. Jacob Lemieux, an infectious disease doctor at Massachusetts General Hospital, said the surge in cases would not match the first Omicron spike Americans experienced a year ago. “Is this a category five hurricane?” They said. “No.”

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

White House COVID-19 Response Coordinator Dr. Ashish K. Jha said the Biden administration was monitoring the emergence of XBB.1.5 and urged people to take advantage of existing countermeasures. preliminary studies suggest that bivalent vaccines Should provide good protection against XBB and its descendants. Paxlovid will also be effective in fighting the infection.

“We feel very comfortable that our countermeasures will continue to work,” Dr. Jha said. “But we have to make sure people are using them.”

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

That descendant may already be present, infecting people without taking notice. But worldwide sequencing efforts have fallen so far that the search for the next generation of XBB.1.5 may be delayed. “Because sequencing is becoming less and less available globally, it is difficult for us to track each subtype of omicron,” said Dr. Van Kerkhove.

Scientists have reconstructed the evolution of XBB.1.5 (which some have named Kraken) by considering 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 virus replicated, their genetic material was mixed together. A new hybrid form emerged with genetic material from both viral parents. Virus watchers have named it XBB.

this mix, called recombination, occurs fairly frequently between coronaviruses. During the pandemic, scientists have discovered several recombinant forms of SARS-CoV-2, which causes COVID-19.

Most recombinant SARS-CoV-2 viruses are attenuated within a few weeks or months, unable to outcompete other lineages. On the other hand, XBB got the winning ticket in the genetic lottery. From one parent, it received a set of mutations that helped it avoid antibodies from past infections and vaccinations. From the other parent, it received a different set of mutations that made it even more evolved.

“XBB actually picked up the most likely mutation from those two parents,” said Thomas Peacock, a virologist at Imperial College London. The new combination made XBB one of the most aggressive Omicron subvariants in existence last summer.

Recent experiments suggest that XBB pays a heavy price for its power to evade immunity. The mutation allows it to evade antibodies by changing the shape of a protein, called a spike, that covers its surface. But some of those mutations make it harder for the XBB spike protein to grip tightly to cells — the first step needed for infection.

That loose grip may have reduced XBB’s advantage against other forms of the virus. In late 2022, it jostles with several other Omicron subvariants. In Singapore, XBB caused a boom in October, for example, while remaining scarce in many other parts of the world.

As the XBB multiplied, it mutated into new forms. Early samples of XBB.1.5 were isolated in New York in October. The new subvariant acquired a key mutation, known as F486P.

Peking University’s Yunlong Cao and colleagues tested XBB.1.5 in dishes of cells, comparing how it performed against earlier forms of XBB. The researchers found that the F486P mutation allowed XBB.1.5 to tightly grip cells again. But the new subvariant can still evade antibodies as well as earlier forms of XBB.

Dr. Cao and his colleagues posted their result online on Thursday. The data has not yet been published in a scientific journal.

XBB.1.5 most likely evolved in the northeastern United States, where early samples first recognized and where it remains most common. Once scientists identify it, they can track its evolution.

in ConnecticutFor example, Nathan Grubaugh and his colleagues at Yale University found that by mid-December, other omicron subvariants were declining. Only XBB.1.5 cases were on the rise. Dr. Grubaugh estimates that it is transmitted about 20 percent more often than BQ.1, which has been the dominant form.

“It doesn’t have those signs of a really big wave like we’ve seen before,” he said. “It was not going to come close to last year.”

It is not yet clear how serious the XBB.1.5 infection is compared to other forms of the coronavirus. “It’s serious,” Dr. Grubaugh said. “I just don’t necessarily know if it’s actually more severe than some of the other Omicron offshoots in terms of overall impact.”

XBB.1.5 has already spread to other countries, and is growing rapidly in Germany, Denmark and elsewhere in Europe. But its impact is likely to vary from place to place. In India, for example, it will encounter many people who were infected with its ancestral strain last year, so it may encounter stronger immunity, Dr. Peacock said.

In China, which experienced a major surge of cases in late 2022, its prospects are even more difficult to predict. For most of the pandemic, China almost never shared virus sequences with international databases. Collaboration has increased over the past few weeks, but the databases still may not reflect the state of the game in the country.

Much of XBB.1.5’s advantage in the United States comes from its ability to evade existing immunity, including that against other Omicron subvariants. In China where immunity is low, there may not be that edge. Dr. Peacock speculated that after other variants spread to China, it could be XBB.1.5’s turn to rise.

Dr. Wenselaers said the spread of XBB.1.5 outside China made him skeptical that restrictions on Chinese travelers would keep cases down. “It’s kind of pointless,” he said. “It would be better to make sure the elderly are well vaccinated.”

As XBB.1.5 spreads, it continues to mutate, and experts believe it may become even better at evading antibodies.

Scientists are scanning the new sequences already being uploaded to an international database GISAID In the hope of finding an upgraded version of XBB.1.5. But their job is getting harder as governments back away from systematic efforts. “Worldwide, sequencing has taken a real hit,” Dr. Peacock said.

The United States, which once lagged behind other countries, has managed to maintain a fairly robust sequencing effort. Without it, Dr. Peacock said, XBB.1.5 could have stayed under the radar for much longer. If a next generation of XBB.1.5 is in development somewhere with a bit of sequencing, it won’t be known for some time to come.

Dr. Lemieux said holding back on sequencing was a mistake, given how many infections and deaths the virus is still causing. “It’s a part of public health,” he said.

And Dr. Peacock said that XBB.1.5 demonstrated that the evolution of the coronavirus won’t be slowing down anytime soon. “Give it another two years, and maybe we can reevaluate where it’s at,” he said.

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