Replication of all recombinant viruses in Vero cells were examined by performing both one-step and multistep growth kinetics at a multiplicity of infection (MOI) of 1 and 0.001, respectively. Interestingly, although replacement of the S gene resulted in an alteration of the replication kinetics and efficiency, leading to similar growth curve patterns between viruses carrying the identical S gene sequence, it did not fully reverse the capability of virus yield, at least in Vero cells (Figure 1C).