A W.H.O. collaborative study of in vitro and in vivo methods for the assay of yellow fever vaccines.

An international collaborative study was carried out to compare the plaque assay in monolayer cell cultures with the mouse assay for the potency testing of yellow fever vaccines. Twelve laboratories assayed four preparations of yellow fever virus by both methods. Differences were found between estimates of infectivities obtained by different laboratories using the plaque assay. There was also large variation between the estimates of mouse LD50 obtained by different laboratories. However, expressing potencies of the preparations relative to a common preparation greatly reduced the variation between laboratories for both the plaque and the mouse tests; it also resulted in remarkably close agreement between the methods.