SARS-CoV-2 Delta variant excels at membrane fusion, but not immune evasion

The SARS-CoV-2 Delta variant has become the dominant strain worldwide. It is around twice as transmissible as its ancestral strain, with a shorter incubation period and higher viral load during infection. Now, Bing Chen and colleagues show that mutations in spike protein of Delta allow for faster membrane fusion than Alpha, Beta, Gamma and Kappa variants, and that Delta is more efficient at infecting cells with very low expression of the ACE2 entry receptor. However, the mutations found in the Delta variant had less impact on its sensitivity to neutralizing antibodies compared to those of the Gamma and Kappa variants. Neutralizing antibodies predominantly target the N-terminal domain (NTD) or the receptor binding domain (RBD) of the spike protein. The authors found different arrangements of the antigenic surface of the NTD in the different variants, but only local changes in the RBD, indicating that therapeutic antibodies or universal vaccines should be targeted at the latter.