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The Covid-Omicron XBB.1.5 variant is quickly becoming dominant in the US because it is highly immune-avoidable and appears to be more effective at binding to cells than related subvariants, scientists say.
According to data released Friday by the Centers for Disease Control and Prevention, XBB.1.5 now accounts for about 41% of new cases nationwide in the US and has nearly doubled in prevalence over the past week. The subvariant more than doubled as a proportion of cases each week through December 24. In the past week it has almost doubled from 21.7% prevalence.
Scientists and public health officials have been closely monitoring the family of XBB subvariants for months, as the strains harbor many mutations that could make the Covid-19 vaccines, including the Omicron boosters, less effective and cause even more breakthrough infections.
XBB was first identified in India in August. It quickly became dominant there, as it did in Singapore. Since then it has evolved into a family of subvariants including XBB.1 and XBB.1.5.
Andrew Pekosz, a virologist at Johns Hopkins University, said XBB.1.5 differs from its family members because it has an additional mutation that makes it bind to cells better.
“The virus needs to attach tightly to cells to be able to enter more efficiently, and that might help the virus infect people a little more efficiently,” Pekosz said.
Yunlong Richard Cao, a scientist and assistant professor at Peking University, on Tuesday posted data on Twitter suggesting that XBB.1.5 not only bypasses protective antibodies as effectively as the XBB.1 variant, which was highly immune avoidable, but can also bind better to cells through a key receptor.
Columbia University scientists warned in a study published earlier this month in the journal Cell that the rise in subvariants such as XBB “further impairs the effectiveness of current COVID-19 vaccines and leads to an increase in breakthrough infections and re-infections. “
The XBB subvariants are also resistant to Evusheld, an antibody cocktail that many people with compromised immune systems rely on to protect against Covid infection because they don’t show a strong response to the vaccines.
The scientists described the resistance of the XBB subvariants to antibodies from vaccination and infection as “alarming”. The XBB subvariants were even more effective at evading protection from the Omicron boosters than the BQ subvariants, which are also highly immune evasive, the researchers found.
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dr David Ho, an author of the Columbia study, agreed with the other scientists that XBB.1.5 probably has a growth advantage because it binds to cells better than its XBB relatives. Ho also said XBB.1.5 is about as immune-preventable as XBB and XBB.1, which were two of the subvariants most resistant to protective antibodies from infection and vaccination to date.
dr Anthony Fauci, who is stepping down from his role as senior White House medical adviser, has previously said that the XBB subvariants reduce the protection that the boosters offer against infection “severalfold”.
“You can expect some protection, but not optimal protection,” Fauci told reporters during a November White House briefing.
Fauci said he was heartened by the case of Singapore, which had a large spike in infections from XBB but hospitalizations did not increase at the same rate. Pekosz said XBB.1.5 combined with vacation travel could cause cases to spike in the US. But he said the boosters appear to prevent serious illnesses.
“It looks like the vaccine, the bivalent booster, with these variants offers sustained protection against hospitalization,” Pekosz said. “This really underscores the need to get booster vaccines, particularly in vulnerable populations, to provide sustained protection against serious diseases with these new variants.”
Health officials in the US have repeatedly urged older people in particular to make sure they are up to date with their vaccines and seek treatment with the antiviral Paxlovid if they have a breakthrough infection.