XBB.1.5 on its way to being predominant COVID-19 subvariant

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Graphic: COVID-19 molecule with distinctive spike proteins, says "XBB.1.5" over molecule.

Since it was first identified in late October (in New York), Omicron subvariant XBB.1.5 has rapidly outcompeted other Omicron subvariants and is believed to now represent more than 70% of cases in the northeast United States. XBB.1.5 comprised an estimated 27.6% of COVID-19 cases in the United States during the first week of 2023. At the same time, the Omicron subvariant BQ.1.1 represented an estimated 34.4% of cases, but health experts expect XBB.1.5 to overtake BQ.1.1 soon and become the predominant variant in the U.S. and around the world.

Where did XBB.1.5 come from? 

XBB.1.5 can be thought of as a “great grandchild of Omicron.” It probably started when a person was infected with Omicron subvariants BA.1.10.1 and BA.275 simultaneously. When the two subvariants replicated in that person’s body, they swapped genes to create a new subvariant—XBB. And XBB.1.5 is an offspring of XBB. It’s the latest example of the COVID-19 virus (SARS-CoV-2) mutating to become more contagious. 

XBB.1.5 possesses a number of mutations including one that is very good at binding to human respiratory cells and replicating. This adaptation has helped make it likely the most contagious variant of COVID-19 to date. 

“This new subvariant is a perfect example of how the SARS-CoV-2 virus is capable of adapting to selective pressure through mutations and re-combinations with earlier variants,” said Xuan Qin, Ph.D., director of clinical microbiology at Oregon Health & Science University (OHSU) and a professor of pathology in the OHSU School of Medicine. 

 Is XBB.1.5 more severe? 

Although it is highly contagious, there is no evidence to suggest XBB.1.5 causes more severe illness than previous COVID-19 strains. Additionally, the immunity from vaccinations and previous infections helps reduce the chance of severe disease. The new bivalent booster will provide protection against XBB.1.5 and staying up to date on your COVID-19 vaccinations is still the best way to protect yourself from severe disease. 

Because XBB.1.5 is more contagious, leading to more cases, COVID-19-related hospitalizations are expected to rise. The latest statewide forecast from OHSU anticipates the arrival of XBB.1.5 will offset previously expected declines in COVID-19-related hospitalizations. 

As COVID-19 continues to spread, it will also continue to mutate and create new variants or subvariants. Omicron has dominated for more than a year now.  

“This variant will not be the last we see,” said Bill Messer, M.D., Ph.D., associate professor of molecular microbiology and immunology, and medicine (infectious diseases) in the OHSU School of Medicine. “We are going to have to figure out how we ‘greet’’ the new variants, much like we look to see what influenza will do each year: Are the changes big, or small? Will the vaccine work? What else should I be doing to protect myself, my family and my friends and colleagues from getting seriously ill? Masking, hand hygiene, covering your mouth when you cough, staying home when you’re ill and getting vaccinated all will be important.”