Despite intensive public health efforts to grind the COVID-19 pandemic to a halt, the recent emergence of the highly transmissible, extensively drug-resistant and profoundly immune system-evading XBB.1.5 SARS-CoV-2 subvariant is putting the global community on edge.

What is XBB.1.5?
In the naming convention for SARS-CoV-2 lineages, the prefix “X” denotes a pedigree that arose through genetic recombination between two or more subvariants.

The XBB lineage emerged following natural co-infection of a human host with two omicron subvariants, namely BA.2.10.1 and BA.2.75. It was first identified by public health authorities in India during summer 2022. XBB.1.5 is a direct descendent, or more accurately, the “fifth grandchild” of the original XBB subvariant.

How does XBB.1.5 differ from omicron?

XBB.1.5 is one of many omicron subvariants of concern that have appeared on the global pandemic scene since the onset of the first omicron wave in November 2021. In contrast to other descendants of the original omicron variant (known as B.1.1.529), XBB.1.5 is a mosaic subvariant that traces its roots to two omicron subvariant lineages.

Where is XBB.1.5 prevalent?

According to the World Health Organization, XBB.1.5 is circulating in at least 38 countries, with the highest prevalence in the United States, where it accounts for approximately 43 per cent of COVID-19 cases nationwide. Within the U.S., there is wide geographic variation in the proportion of cases caused by XBB.1.5, ranging from seven per cent in the Midwest to over 70 per cent in New England.

XBB.1.5 has also been officially reported by governmental agencies in Australia, Canada, the European Union, Japan, Kuwait, Russia, Singapore, South Africa and the United Kingdom. Real-time surveillance data reveals that XBB.1.5 is rapidly spreading across the globe and will likely become the next dominant subvariant.

XBB.1.5 has also been detected in municipal wastewater systems in the United States, Europe and other places.

Are current mRNA vaccines effective against XBB.1.5?

XBB.1.5 and XBB.1 are the omicron subvariants with the greatest immune-evasive properties. Therefore, one of the most contentious issues surrounding XBB.1.5 relates to the degree of protection afforded by currently available mRNA vaccines, including the latest bivalent booster formulations.

Researchers from the University of Texas determined that first-generation and bivalent mRNA booster vaccines containing BA.5 result in lackluster neutralizing antibody responses against XBB.1.5. A report (yet to be peer reviewed) from investigators at the Cleveland Clinic found that bivalent vaccines demonstrate only modest (30 per cent) effectiveness in otherwise healthy non-elderly people when the variants in the vaccine match those circulating in the community.

Furthermore, some experts believe the administration of bivalent boosters for the prevention of COVID-19 illness in otherwise healthy young individuals is not medically justified nor cost-effective.

In contrast, public health experts from Atlanta, Ga. and Stanford, Calif. reported that although the neutralizing antibody activity of bivalent booster vaccines against XBB.1.5 is 12 to 26 times less than antibody activity against the wild-type (original) SARS-CoV-2 virus, bivalent vaccines still perform better than monovalent vaccines against XBB.1.5.