It is a sub-variant that "over-performs".

The XBB 1.5 mutation of the Omicron variant of Sars-CoV-2 is spreading at very high speed in the United States.

In barely a month, this strain has gone from less than 5% of the contaminations observed to more than 40%.

“We had not seen such a speed of spread since the emergence of Omicron at the end of 2021. It is impressive”, underlines Lawrence Young, virologist at Warwick Medical School.

A report shared by the World Health Organization (WHO) which qualified, Wednesday January 4, XBB 1.15 of "variant of Omicron the most transmissible to date".

Mutation of a recombination of subvariants of Omicron

This strain is a version 3 or even 4.0 of the Sars-CoV-2 virus, since it is not only a sub-variant of Covid-19.

“It is a mutation of a recombination of Omicron variants”, specifies Jonathan Stoye, virologist and research manager at the Francis Crick Institute in London.

Indeed, the "XBB" line from which it comes has the particularity of being a recombination of two Omicron variants.

Unlike strains which are the result of mutations of a viral "parent" - the most frequent -, "recombination generally occurs within an individual infected by several variants which merge together", summarizes Lawrence Young.

And the "1.5" strain adds a new mutation to this recombination which seems to be at the origin of its express transmissibility.

XBB 1.5 actually has two specificities: this mutation is very resistant to vaccines as well as to the immunity developed after contamination with Omicron, and this variant "clings more easily and firmly to the cells of the human body to infect them" , notes Lawrence Young.

Its resistance is a legacy.

"It was already present in the other XBB sub-variants and the new mutation did not alter it", underlines Simon Clarke, researcher in infectious diseases at the University of Reading.

The big change, which is located at position 486 of the virus's genetic code, "transforms the Spike protein - the famous key that allows the virus to open the cell door - so that it more easily attaches to the receptor" , continues the British researcher.

Hence an XXL transmissibility.

Indeed, "if you have a virus that infects cells more easily, you need less pathogen to infect a person", summarizes Simon Clarke, who specifies however that there may also be other factors to the work favoring the spread (for example if this variant causes more coughing).

XBB 1.5 conquering the world?

XBB 1.5 therefore presents a profile that seems to guarantee it a worldwide destiny.

The German Minister of Health, Karl Lauterbach, has also called for the greatest vigilance vis-à-vis this virus.

“I hope we will get through the winter before such a variant spreads to us,” he said, referring to a potential overload of the hospital environment already very busy with cases of seasonal viruses like the flu. 

In the UK too, the trajectory of XBB 1.5 is beginning to resemble that of the US.

"There is no reason for the rest of the world to escape this variant", assures Jonathan Stoye.

But it may not be as quick to establish itself everywhere.

“We must also take into account the immunological profile of the populations”, underlines Lawrence Young.

In the regions of North America where XBB 1.5 has spread the fastest, it is likely that the mix of variants that were circulating before the appearance of this new strain produced only weak immunity against this type of variant to those that had already contracted Covid-19, believes this virologist.

In addition, "the vaccine booster campaign in the United States has been less effective than in other countries, especially in Europe", underlines Lawrence Young.

As a result, even if this new strain escapes vaccine immunities more easily (including specific boosters for the Omicron variant), the fact remains that in Europe "there are more protective barriers", notes Jonathan Stoye.

The big unknown remains the virulence of XBB 1.5.

The wave of contamination currently raging in the United States has led to an increase in hospitalizations of just over 4% compared to last week.

The same is true for admissions to intensive care units, which are up 9%.

No wonder: "A variant that circulates faster, will also more easily and quickly reach the people most at risk", notes Lawrence Young.

Above all, the regions most affected by XBB 1.5.

do not experience a particular worsening of the health situation compared to the American states where the new variant is not yet in the majority.

"Current data in the United States suggests that this new mutation is no more dangerous than other versions of the Omicron variant", advances Simon Clarke, while indicating that it is for the moment a sample still low case.

Even if XBB 1.5 turns out to be no more virulent than its cousins, that's no reason to treat it casually.

A virus that spreads so quickly has every chance of generating sub-variants of sub-variants in turn.

Until one of these creations actually turns out to be more virulent than all the others.

For Jonathan Stoye, XBB 1.5 "must really be a reminder that the pandemic is not over yet".

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