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XEC – first identified in Germany in early August – is a recombination of two JN.1 lineages, KS.1.1 and KP.3.3. In the United States, the KP.3.1.1 variant is still dominant and increasing. It accounts for an estimated 57.2% of SARS-CoV-2 viruses, according to the Centers for Disease Control and Prevention (CDC) in its latest projections. However, a steady increase in XEC viruses was also detected, accounting for approximately 10.7% of the sequenced samples.

Kei Sato, PhD, who leads the lab, said in comments on the study that modeling of the group's phylogenetic epidemic dynamics, based on surveillance from five countries, suggests that the reproductive number of XEC is greater than that of KP.3.1. 1, the currently dominant virus worldwide. The reproduction number is the average number of additional cases that each case produces in a susceptible population.

Increased infectivity, greater immune evasion

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

Experiments with XBB.1.5 and JN.1 sera to assess breakthrough infection revealed that the neutralization titers for 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 greater immune evasion.

Sato wrote that XEC's higher reproduction number compared to KP3.1.1 was partly due to its more robust resistance to KP.3.3.