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XEC – first identified in Germany in early August – is a recombinant of two JN.1 lineages, KS.1.1 and KP.3.3. In the United States, the KP.3.1.1 variant remains dominant and emerging, accounting for an estimated 57.2% of SARS-CoV-2 viruses, the Centers for Disease Control and Prevention (CDC) said in their latest projections. However, it also noted a steady increase in the number of XEC viruses, which account for approximately 10.7% of the samples sequenced.

Commenting on the study on The group’s phylogenetic epidemic dynamics models, based on surveillance from five countries, suggest that the reproduction number of XEC is greater than that of KP.3.1.1, which is currently the dominant virus in the world. The reproduction number is the average number of additional cases generated by each case in a susceptible population.

Increased infectivity, more immune evasive

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

Experiments using XBB.1.5 and JN.1 sera to assess breakthrough infections showed that neutralization titers for XEC and KP.3.1.1 were comparable. 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, indicating higher immune evasion.

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