In Vero cells, rhMPV/ΔM2-2 plaques were less than 50% the size of rhMPV plaques (Figure 2A) . A 4-day multi-cycle growth curve was performed in Vero cells, a cell-line used for production of live vaccines, to compare the replication kinetics of rhMPV/ΔM2-2 and rhMPV. Data for the replication curves of these viruses were collected from three independently performed infections. The peak titer of rhMPV/ΔM2-2 in Vero cells was 7.22 +/-0.16 log 10 PFU/ ml, which was not significantly lower than the 7.52 +/-0.29 log 10 PFU/ml titer achieved by rhMPV ( Figure 2B ). However, the plaque size of rhMPV/ΔM2-2 was markedly diminished compared to rhMPV. Thus, hMPV M2-2 is dispensable for replication in Vero cells.