The viral titer growth rate of the H275Y mutant was 0:83+ 0:02h {1 , slightly greater than that of the WT which was 0:75+0:04h {1 . Thus, it would appear from the viral titer growth rate alone, that the H275Y mutant has a replicative advantage over the WT strain, in contrast with the findings using the plaque velocity extracted from the plaque assay alone.