The Covid Omicron XBB.1.5 variant is quickly becoming dominant in the US because it is highly immune evasive and appears to be more effective at binding to cells than related subvariants, scientists say.
XBB.1.5 now represents about 41% of new cases nationwide in the US, nearly doubling its prevalence in the past week, according to data released Friday by the Centers for Disease Control and Prevention. The subvariant has more than doubled as a percentage of cases each week through December 24. In the past week, it has nearly doubled from 21.7% prevalence.
Scientists and public health officials have been closely watching the XBB subvariant family for months because the strains have many mutations that can make 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 well as in Singapore. It has since grown into a family of sub-variants, including XBB.1 and XBB.1.5.
Andrew Pekosz, a virologist at Johns Hopkins University, said XBB.1.5 differs from its relatives because it has an additional mutation that allows it to better bind to cells.
“The virus needs to bind tightly to cells to get in more efficiently and that could help the virus infect people a little bit more efficiently,” 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 XBB.1 variant, which was highly immune evasive, but also better is in binding to cells through a key receptor.
Columbia University scientists warned in a study published earlier this month in the journal Cell that the emergence of subvariants such as XBB “could further compromise the efficacy of current COVID-19 vaccines and result in a surge of breakthrough infections and new infections.”
The XBB subvariants are also resistant to Evusheld, an antibody cocktail that many people with weak immune systems rely on for protection against Covid infection because they don’t mount 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 against the omicron boosters than the BQ subvariants, which are also highly immune evasive, the scientists found.
Dr. David Ho, an 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 about as immune-evasive as XBB and XBB.1, which were two of the subvariants most resistant to protective antibodies against infection and vaccination so far.
Dr. Anthony Fauci, who is leaving his role as chief medical adviser to the White House, has previously said that the XBB sub-variants reduce the protection the boosters provide against infection “many folds”.
“You could expect some protection, but not the optimal protection,” Fauci told reporters at a briefing at the White House in November.
Fauci said he was encouraged by the case of Singapore, which had a large surge in infections from XBB, but did not see hospitalizations increase at the same rate. Pekosz said XBB.1.5, when combined with holiday travel, could increase the number of cases in the US. But he said the boosters appear to prevent serious illness.
“It’s like the vaccine, the bivalent booster provides continued protection against hospitalization with these variants,” Pekosz said. “It really highlights the need to boost vulnerable populations in particular to provide continued protection against serious disease with these new variants.”
Health officials in the US have repeatedly called on the elderly in particular to ensure they are up to date on their vaccines and that they are treated with the antiviral drug Paxlovid if they have a breakthrough infection.