TMPRSS2 and infection: We found that this system led to fusions being present early post-infection by cell-free virus (see Supplementary Movie S1 for timelapse microscopy of uninfected Vero-TMPRSS2 cells and Movie S2 for D614G infected cells) and we could quantify fusions at 10 h post-infection, therefore reducing the effects of replication differences between strains.