A new omicron subvariant called XBB.1.5 has now become the dominant Covid-19 strain and is spreading rapidly in the US, with cases also recorded in countries across Europe, including France, Netherlands, Italy, and the UK.
Experts warn that the new strain is the most transmissible strain yet due to its immune-evasive properties. As the world enters a new year, testing and monitoring for the new variant will be an increasingly important way to contain the spread of infection.
One of the latest offshoots of the highly transmissible Omicron variant, which led to a surge in hospital admissions in the UK last winter, XBB.1.5 (dubbed “Kraken”) was first detected in the United States in October 2022, according to the European Centre for Disease Prevention and Control (ECDC). Currently, experts are primarily concerned with its ability to quickly spread and overtake other strains of Omicron. So far, the US is experiencing the most significant growth of the new variant - the proportion of Covid-19 infections due to XBB.1.5 were doubling almost every week at the end of December, and the new variant now accounts for about 40% of all Covid infections in the country. To date, XBB.1.5. has also been detected in 29 other countries worldwide – and makes up at least 4% of viruses being sequenced in the UK (1).
In a press conference early this year, experts from the World Health Organization (WHO) said XBB.1.5. is “the most transmissible subvariant detected yet”, with evidence showing that it is more immune evasive and better at infecting than other subvariants of Covid-19. This is due to an unusual mutation known as F486P, which is making it harder for the antibodies people have already developed from Covid vaccinations or previous infections to successfully neutralize the virus.
While there may be many different versions of SARS-CoV-2 floating around, all of them work essentially the same way: they worm their way into our cells using a “key” on the outside of the virus called a spike protein. Sometimes a mutation generates a more efficient key meaning the virus is able to enter cells more easily.
There are only slight differences in the genetic codes of these viruses, but the mutations in XBB.1.5 are a big problem. XBB.1.5 evolved from the XBB variant of Omicron and is a fusion of two different BA.2 variants. The XBB variant has a different mutation at the same position, which makes it good at evading immune defences too; however, the virus cannot latch on to human cells as effectively, which actually makes it less infectious.