“XBB is a hybrid version of two strains of the BA.2 form of Omicron,” explains Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. It’s currently “spreading efficiently in Singapore,” he adds.
The variant was first detected in August 2022 in India, and has been detected in more than 17 countries since then, including Australia, Bangladesh, Denmark, India, Japan and the U.S., per Singapore’s
Ministry of Health.
XBB is thought to have the best ability to evade antibody protections of these newly emerged COVID variants, according to a
pre-print study from researchers in China. That study said that the new strains of Omicron, and XBB in particular, “are the most antibody-evasive strain tested, far exceeding BA.5 and approaching SARS-CoV-1 level.” (SARS-CoV-1, in case you’re not familiar with it, is the strain of coronavirus that causes
SARS, a respiratory virus that can cause severe illness.)
Meaning, the vaccine and having previously had COVID-19 are not thought to offer the same level of protection against XBB as they have with previous strains of COVID-19. Antibody drugs like Evusheld and bebtelovimab may also not be very effective against XBB, the
pre-print study says.
“These variants are evolving to evade protection,” Dr. Russo says. The bivalent booster is “likely going to be protective against severe disease” with XBB, but will be “imperfect against preventing infection," Dr. Russo says.
Don’t panic, though. “When it comes to evasion of vaccine protection, it’s important to recognize that vaccine protection is not all or none,” Dr. Adalja says. “Even with immune-evasive variants, vaccine protection against what matters most—severe disease—remains intact.”