Quantitation of infectious yellow fever virus by plaque assay and TC-LDA.

aPlaque-forming yellow fever virus (YFV) strain YFV-17D-213-77 at Vero cell passage number 0, 1, 2 (P0, P1, and P2, respectively) and YFV-FNV (French neurotropic vaccine, Vero cell Passage 1) were quantitated by plaque assay or by flow cytometry-based tissue culture-limiting dilution assay (TC-LDA). The coefficient of variation (CV) indicates the inter-assay variability of the two independent approaches to virus quantitation based on 3–5 independent experiments. The variability (% CV) does not appear to be related to passage number since the YFV-FNV P1 stock has a similar TC-LDA CV to the YFV-17D P2 stock. YFV-Dakar was not included in these analyses since it did not form plaques on Vero cells (data not shown) and was not detectable by focus forming assay (Figure 1).ifu; Infectious Unit.