Fig. 1. More efficient membrane fusion by the Delta variant than other variants.

(A) Time course of cell-cell fusion mediated by various full-length S proteins, as indicated by the labels, with HEK293 cells with no exogenous ACE2. (B) Cell-cell fusion mediated by various full-length S proteins with HEK293 cells transfected with low levels (0 to 0.25 ng) of ACE2 expression constructs. (C) Time course of infection HEK293-ACE2 cells by MLV-based, pseudotyped viruses by various SARS-CoV-2 variant S constructs containing a CT deletion in a single cycle. Infection was initiated by mixing viruses and target cells, and viruses were washed out at each time point as indicated. The full time course and concentration series are shown in fig. S3. The experiments were repeated at least three times, with independent samples each giving similar results.