COVID-19: which Omicron variant is the most contagious so far?

A subvariants highly contagious Omicron is now the dominant strain spreading in the United States. Early data suggest that the XBB.1.5 it is more invasive than other variantsefficiently dodging previously acquired immunity and raising concerns among public health officials about a possible winter snap in the northern hemisphere.

In December alone, XBB.1.5 went from causing less than 1% of confirmed COVID-19 cases in the United States to more than 40%, rapidly outperforming other variants. In the northeastern states it caused more than 75% of all cases during the week beginning December 25, although there is no data yet to indicate whether it causes a more serious illness.

It is the most transmissible subvariant that has been detected so far,” said Maria Van Kerkhove, technical lead for the response to COVID-19 at the World Health Organization, during a press conference. Scientists estimate that someone infected with XBB.1.5 can cause 60% more infections than a person infected with their parent strains.

XBB.1.5 was first detected in New York and Connecticut, in the United States, in late October, but since then it has been detected in at least 29 other countries. While it accounts for less than 5% of current cases worldwide, it appears to be doubling within eight to 15 days, making it the fastest-spreading variant of SARS-CoV-2 until now.

In fact, XBB.1.5 could be much more widespread, according to Kerkhove, since it’s hard to say, since genomic sequencing efforts to monitor SARS-CoV-2 have declined worldwide.

Where does XBB.1.5 come from?

XBB.1.5 descends from the XBB.1 variant, which arose from XBB, the merger of two Omicron BA.2 variants. His grandparent strain XBB and its parent strain XBB.1 were responsible for a sudden increase in COVID-19 cases in parts of Asia in October and November 2022. They were able to escape immunity conferred by previous infections and bivalent boosters that were specifically designed to block Omicron variants, according to a report published in Nature in December. In Singapore, the XBB subvariants caused a large number of infections and intercurrent reinfectionsalthough hospitalization rates remained low.

How is the new variant different?

To the variants XBB.1.5 introduces a new mutation at position 486 of the spike protein, which is what the virus uses to bind to the ACE2 receptor protein on cells of the human respiratory tract. mathematical models predicted that a change in the spike protein at this position would allow a variant to escape previous antibodies. Those predictions appear to be correct.

“The mutation brings about increased binding of ACE2 while maintaining the extremely high immunity evacuation capacity of XBB.1,” says Yunlong Cao of Peking University in China, based on his preliminary research exploring enhanced contagion of XBB.1.5, which has not yet been peer reviewed.

Cao’s preliminary investigation also shows what some monoclonal antibody therapieslike Evusheld and Bebtelovimab, do not block XBB.1.5although Sotrovimab provided weak protection.

Does bivalent reinforcement protect against XBB.1.5?

It is too soon to know how the current bivalent reinforcementwhich targets Omicron’s earlier BA.4 and BA.5 strains, as well as the original coronavirus, would protect against XBB.1.5. However, a recent investigation on the parental strains of XBB.1.5 led by Mehul Suthar, an immunologist at Emory University School of Medicine, shows that people who received the bivalent boosteras well as those recently infected with an Omicron variant, have slightly higher levels of protective antibodies.

Bivalent boosters seem to be working like they’re supposed to,” Suthar says, changing our immunity to Omicron variants. He speculates that bivalent reinforcement would provide some protection against XBB.1.5, based on its similarity to other Omicron variants. But his research also suggests that even bivalent booster may not block advanced XBB.1.5 infections.

The Centers for Disease Control and Prevention (CDC) estimates that los bivalent boosters reduced COVID-19 hospitalizations in more than 90% until November, which is the latest data available. This means that even if the boosters do not prevent infections, they can still save lives.

“It’s hard to say anything about the pathogenicity of XBB.1.5,” says Kei Sato, a virologist at the University of Tokyo, since there is no data yet. However the preliminary work de Sato, not yet peer-reviewed, on the original XBB strain has shown that the virus adheres more tightly to human cells than other Omicron variantssuch as BA.2.75 and BA.2, suggesting that it could cause more severe disease.

But when the hamsters were infected with XBB, disease symptoms were no worse than those caused by BA.2.75. “We still don’t have any insights to explain this discrepancy,” says Sato.

Will XBB.1.5 prolong the pandemic?

Las hospitalizations for COVID-19 have increased by 17% in the past week, according to the latest CDC data, leading scientists to worry that even Without causing more severe illness, the highly contagious XBB.1.5 could worsen the course of the pandemic which has so far lasted three years. Large numbers of COVID-19 cases may still overwhelm hospitals, where resources are already running low due to the high number of RSV and flu respiratory infections.

People older than 65 are the most vulnerable to complications due to COVID-19. However, just a little more than one-third of these Americans have received the bivalent booster, leaving them vulnerable to XBB.1.5. When it comes to the US population, almost 85% of those eligible have not received the bivalent vaccine.

“When infections are high, many people will be affected,” says Marc Veldhoen, an immunologist at the Institute of Molecular Medicine in Lisbon, Portugal. “More infections carry more risk, more likely symptoms and unfortunately, high circulation means that many [personas] vulnerable will get infected and we will see more people in hospitals.”

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