What do we know about new COVID variant XBB.1.5?

A new Omicron variant 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.

The XBB.1.5, which is a descendant of the Omicron XBB subvariant, has started to rise in parts of Europe, according to the World Health Organization (WHO).

Two other Omicron variants – BA.5.2 and BF.7 – were the dominant strains behind a sharp increase in COVID-19 cases in China. Beijing has said at least 60,000 people have died in the past month.

Here is what we know about XBB.1.5:

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.

Is XBB.1.5 more infectious?

On January 4, Maria Van Kerkhove, WHO’s technical lead on COVID-19, said XBB.1.5 is the “most transmissible subvariant detected yet”.

“The reasons for this are the mutations that are within this subvariant of Omicron allowing this virus to adhere to the cell and replicate easily,” she added.

SARS-CoV-2 coronavirus has continued to mutate since it emerged three years ago. Scientists have struggled to contain the coronavirus as it has constantly mutated – meaning it changed its genetic code. Mutations also meant the virus could evade immune systems and vaccines.

According to experts, a mutation known as F486p gives Kraken a greater advantage than XBB, allowing it to better attach to the ACE2 receptor in cells – a process through which COVID-19 spreads in humans.

Symptoms of the subvariant are similar to previous Omicron strains including congestion, runny nose and fever.

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.

Ryan Gregory of the University of Guelph in Canada told Al Jazeera that it can be difficult to determine if a variant is more severe because there were “multiple variants circulating in the same place”.

“In the northeastern USA, where XBB.1.5 is most prevalent, there is also still a lot of BQ.1 and BQ.1.1 [Omicron subvariants] circulating. There isn’t any indication yet that it is likely to be more severe, nor that it will cause a wave the size of the first Omicron last winter,” the professor of evolutionary biology said.

“Also, as we learned with the first Omicron wave, less virulent [than Delta] did not mean fewer hospitalisations and deaths – 2022 was the deadliest year of the pandemic in Canada and the UK, for example, and that was all Omicron.”

Moreover, Gregory said the situation with the variants had changed in the past several months.

“The existence of an entire ‘variant soup’ rather than one variant rising to dominance and then being replaced by a single subsequent variant looks like it will mean a higher sea level with high and low tide instead of a series of tsunamis,” he added.

Will vaccines work?

Like its parent XBB, Kraken has strong immune evasive properties compared with previous Omicron sub-lineages, according to scientists.

Guelph professor Gregory told Al Jazeera that all indications were that the bivalent vaccine – which includes a combination of the original virus and the Omicron variant – continues to provide “good protection” including XBB strains in terms of preventing severe acute disease.

“They don’t stop transmission, and it is less clear how well they protect against the risk of long COVID, but they do work to reduce hospitalisations,” he added.

“That is, if people get boosted – in many places, booster rates have been very low.”

Syra Madad, senior director of the System-wide Special Pathogens Program at NYC Health + Hospitals said in addition to staying up to date with vaccines, it was important to “layer on protection, as no one layer is 100 percent”.

“This includes wearing a high-quality mask in indoor settings with poor ventilation or when in crowded spaces,” she told Al Jazeera.

“With three years under our belt with this pandemic, we can all confidently say masks have worked and are a great tool to reduce one’s risk of getting infected with COVID-19.”