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According to the scientists, the Covid omicron XBB.1.5 variant is rapidly becoming dominant in the United States because it is highly immune-evasive and appears to be more effective at binding to cells than related subvariants.
XBB.1.5 now accounts for about 41% of new cases nationwide in the US, nearly doubling in prevalence over the past week, according to data released Friday by the Centers for Disease Control and Prevention. The subvariant more than doubled its share of cases each week through December. 24. Last week, it almost doubled from 21.7% prevalence.
Scientists and public health officials have been closely monitoring the XBB subvariant family for months because the strains have many mutations that could make Covid-19 vaccines, including omicron boosters, less effective and cause even more breakthrough infections .
XBB was first identified in India in August. It quickly became dominant there, as well as in Singapore. It has since 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 is different from its family members because it has an additional mutation that makes it better at binding to cells.
“The virus has to bind tightly to cells to be more efficient at getting in, and that could help make the virus a little more efficient at infecting people,” Pekosz said.
Yunlong Richard Cao, a scientist and assistant professor at Peking University, posted data on Twitter on Tuesday indicating that XBB.1.5 not only evades protective antibodies as effectively as the highly immune-evasive XBB.1 variant, but which also binds better to cells via a key receptor.
Columbia University scientists, in a study published earlier this month in the journal Cell, warned that the rise of subvariants such as XBB could “further compromise the efficacy of current COVID-19 vaccines and lead to to an increase in breakthrough infections as well as re-infections”.
The XBB subvariants are also resistant to Evusheld, a cocktail of antibodies that many people with weak immune systems rely on to protect themselves from Covid infection because they don’t make a strong response to vaccines.
The scientists described the resistance of the XBB subvariants to antibodies from vaccination and infection as “alarming”. The scientists found that the XBB subvariants were even more effective at circumventing the protection of omicron enhancers than the BQ subvariants, which are also highly immune evasive.
Dr. David Ho, author of the Columbia study, agreed with the other scientists that XBB.1.5 likely has a growth advantage because it binds to cells better than its XBB relatives. Ho also said that XBB.1.5 is as immune evasive as XBB and XBB.1, which until now were two of the most resistant subvariants to protective antibodies from infection and vaccination.
Dr. Anthony Fauci, who is leaving his role as White House chief medical adviser, has previously said that XBB subvariants reduce the protection provided by boosters against “multiple” infection.
“You can expect some protection, but not optimal protection,” Fauci told reporters during a White House briefing in November.
Fauci said he was encouraged by the case of Singapore, which had a large increase in XBB infections but did not see hospitalizations rise at the same rate. Pekosz said XBB.1.5, combined with holiday travel, could lead to an increase in cases in the United States, but said the boosters appear to prevent serious illness.
“The vaccine, the bivalent booster appears to provide continued protection against hospitalization with these variants,” Pekosz said. “It really emphasizes the need to get a boost especially in vulnerable populations to provide continued protection against serious disease with these new variants.”
U.S. health officials have repeatedly urged the elderly, in particular, to make sure they are up to date on their vaccinations and to be treated with the antiviral Paxlovid if they have a breakthrough infection.