Reprodueibility of the color test for titration of poliovirus

Titration of poliovaccines: improvement of end-point microassays by means of dose-response curves

The W.H.O.'s titration method for attenuated poliovaccines, using two-fold dilution steps, was assayed concurrently with an alternative method, using four-fold dilution steps. For the two-fold dilution method, the dispersion of the observed proportions of response was very wide, inducing biased estimations for titers and their standard-error by means of simple usual calculation procedures. On the other hand, the use of the four-fold dilution method constantly provides more accurate estimates for the titers and their confidence limits and thus increases the reliability of the titration results. In addition, replicate poliovirus titrations were performed in three cell lines (HEp-2, LLC-MK2 and Vero), applying a restricted experimental design, in order to characterize statistically their sensitivity for poliovaccine titrations, and their consistency for infectivity titers. Means of improving the reliability of live poliovaccine potency estimates are suggested.

Poliovirus and polio antibody assay in HEp-2 and Vero cell cultures

HEp-2 cell cultures were about three to 30 times more sensitive for poliovirus titration than Vero cells. Attenuated strains induced a complete cytopathic effect in HEp-2 but not in Vero cells. For polio antibody titration, HEp-2 and Vero cells were equally suitable. A high degree of sensitivity and reproducibility of virus neutralization was achieved in tests utilizing a low virus dose and serum-virus incubation overnight at 36 degrees C. Staining of infected trays with crystal violet obviated reading of viral CPE under the microscope and expedited the evaluation of larger-scale tests.