Antibody neutralisation of emerging SARS-CoV-2 subvariants: EG.5.1 and XBC.1.6

Although WHO has declared an end to the emergency phase of the COVID-19 pandemic, SARS-CoV-2 has continued to spread and evolve, giving rise to new variants.

These new forms have additional spike mutations that could further compromise protection mediated by antibodies elicited by previous infections, vaccinations, or both. In the past 6 months, two emergent omicron subvariants, EG.5 and EG.5.1, have expanded rapidly worldwide, including in the USA but particularly in China (appendix p 4). EG.5 evolved from the omicron XBB.1.9 subvariant (appendix p 3) and harbours one additional Phe456Leu substitution in the receptor-binding domain (RBD) of the spike protein compared with the recently dominant subvariant XBB.1.5 (appendix p 3). Its immediate descendant, EG.5.1, contains one more mutation, Gln52His in the N-terminal domain (NTD) of the spike protein (appendix p 3). Notably, both EG.5 and EG.5.1 subvariants have the Phe456Leu mutation, which could potentially evade some of the antibodies targeting the class-1 region of the RBD.

Another emergent subvariant is XBC.1.6, a recombinant between the delta variant (B.1.617.2) and the omicron BA.2 subvariant (appendix p 3), which has gained prevalence in Australia (appendix p 4). XBC.1.6 has nine NTD mutations specific to the delta variant in addition to six more mutations compared with the spike protein of BA.2 (appendix p 3). The local growth advantage of XBC.1.6 underscores the need to understand its antibody evasion properties.