XBB.1.5 is causing concerns among scientists as it is spreading like a wildfire across dozens of countries, with the UN health agency calling it the “most transmissible subvariant detected yet”.

In the United States, XBB.1.5 now accounts for more than 40 percent of COVID-19 cases, sharply rising from 2 percent in the first week of December. It is now US’s second-most dominant strain.

What is XBB.1.5?

XBB.1.5 is a sub-variant of the COVID-19 coronavirus Omicron strain, the most transmissible variant of concern (VOC) to date.

The spread of Omicron early last year led to record number of infections worldwide.

Unofficially named Kraken, XBB.1.5 is a sublineage of XBB –  a combination of two strains of the subvariant BA.2. It was first detected in October 2022 in the US.

XBB.1.5 has been circulating in at least 38 countries, including Canada, Australia, Kuwait, Germany and France.

Can XBB.1.5 lead to more serious disease?

Currently, the WHO has said there is no definitive evidence that Kraken will lead to more severe disease than its predecessors.

An internal risk assessment published by the UN agency on January 11 indicated XBB.1.5 did not “carry any mutation known to be associated with a potential change in severity”. However, it asserted that severity assessments were continuing.