Quantification of kinetics of NTVLP production by HA assay. The appearance of virus-like particles was quantified by assaying HA activity in the culture supernatant of LLC-MK2 cells cultured at 32, 37, or 38°C over time (at the time of sampling, the medium was replaced with fresh medium) after infection with SeV18+GFP (A), SeV18+/ΔF-GFP (B), or SeV18+/MtsHNtsΔF-GFP (C) at an MOI of 3.