Although the BA.2 (Omicron) variant is currently the most widespread in France, surveillance continues to determine whether other forms of the SARS-CoV-2 virus are likely to replace it, by overcoming the immunity acquired until to today and causing yet another wave of Covid-19 contamination. Unfortunately, there are still many candidates. Among them, the variants of the “X” family: the so-called XD, XE, XF or even XL.
All have the particularity of being “recombinants”. Viruses that pick up genome segments with different variants in circulation. “For them to appear, first a double contamination is necessary. Then, within the same cell, the virus replication system by polymerase comes into action, passes from one variant to another, copying a end of one, then end of the other”, recently explained Samira Fafi-Kremer, head of the virology laboratory at the Strasbourg University Hospital, to L’Express.
France is watching XD
XD, first detected in January, is still being evaluated by French health authorities due to its rather worrying profile. “The majority of the XD variant genome corresponds to the AY.4 (Delta) sublineage, and a large portion of the S gene (coding for the Spike protein) corresponds to the BA.1 (Omicron) sublineage”, delivers the latest risk analysis related to the variants carried out by Public Health France, and dated March 23. The World Health Organization (WHO) also placed him under surveillance.
During a recent press briefing, Public Health France thus recounted having observed in some patients symptoms closer to those felt because of the Delta variant, such as loss of taste, without more severity. On the other hand, “in terms of interaction with antibodies, the characteristics are closer to Omicron, whose immune escape is known”, indicated an agency official. Fortunately, XD accounted for only 0.1% of infections in week 10 (March 7-13). And it doesn’t seem to settle down since. “There is no worrying epidemiological signal at all for the moment. We have followed it because it really has a very particular profile, but it is especially interesting in fundamental science”, notes the head of the Evolutionary Genomics of RNA Viruses Unit at the Institut Pasteur, Etienne Simon-Lorière, quoted by The Parisian.
On the other side of the Channel, XE questions
Like XD, the XF strain is also a clever mix between the Delta and Omicron variants. It has not been spotted in France, but in the United Kingdom, where sequencing remains very active. However, only 38 cases have been identified, and none since mid-February.
On the spot, it is rather baptized XE – recombinant of BA.1 and BA.2 of the Omicron family – which is the object of a little more attention. As of April 5, some 1,179 sequences of this strain had been analyzed. Published April 8, a British study estimates its growth rate at 20.9% in the first three weeks of March. These data remain to be confirmed, however, and do not in any way predict the ability of XE to supplant the BA.2 variant to which it is ultimately very close.
Finally, we know very little about the strains XG, XH, XJ, XK, spotted sporadically in Europe, or even XL, in South Korea, as well as XN, XP and XQ, if not their composition.
On the other hand, we know with certainty that the Covid-19 still has some surprises in store. “Laboratory experiments suggest that this virus has many solutions to increase its capacity for contagiousness by modifying its spike protein. And there are many other possible mutations on different parts of the virus”, Bruno Canard blew a few days ago at The Express. And the longer the virus continues to circulate, the more combinations it finds.