Germany reports the first case of pox of the new clade

The XEC variant will likely become the predominant SARS-CoV-2 variant worldwide in the near future, researchers at the University of Tokyo-based Sato Lab recently reported in a preprint study.

XEC, first identified in Germany in early August, is a recombinant of two lineages descended from JN.1, KS.1.1 and KP.3.3. In the United States, the KP.3.1.1 variant is still dominant and on the rise, accounting for approximately 57.2% of SARS-CoV-2 viruses, the Centers for Disease Control and Prevention (CDC) said in the his latest projections. However, he also observed a steady increase in XEC viruses, which account for about 10.7% of the sequenced samples.

Commenting on the X study, Kei Sato, PhD, who leads the lab, said the researchers compared the XEC variant with KP.3.1.1, finding that XEC has two tip mutations, compared with KP.3. Modeling of the phylogenetic epidemic dynamics of the group, based on surveillance from five countries, suggests that the reproduction number of XEC is greater than that of KP.3.1.1, which is currently the world’s dominant virus. The reproduction number is the average number of additional cases generated by each case in a susceptible population.

Increased infectivity, more immunoevasive

When the researchers examined the virological properties with pseudovirus experiments, they found that the increased infectivity was due to one of two spike mutations (S:F59S) in XEC.

Experiments with XBB.1.5 and JN.1 sera to assess breakthrough infection found that the neutralization titers of XEC and KP.3.1.1 were similar. However, compared to KP3.3 sera, neutralization against XEC was significantly lower than that of KP.3.1.1, and both mutations significantly increased resistance to KP.3.3 sera, suggesting higher immune evasion .

Sato wrote that the higher reproduction number of XEC compared to KP3.1.1 is partly attributed to the more robust resistance in KP.3.3.