Antigen content of inactivated Newcastle disease oil emulsion vaccines as an in vitro indicator of potency.

The potency of each batch of inactivated Newcastle disease virus (NDV) vaccine is measured in either a vaccination-serology or a vaccination-challenge experiment. We have developed an antigen quantification assay as an in vitro alternative for these currently prescribed in vivo potency assays. The in vitro assay consists of extraction of the viral antigens from the oil emulsion vaccines by isopropylmyristate, followed by quantification of the NDV HN-antigen in an ELISA. The analysis of 20 inactivated vaccines, representing the most common NDV-vaccines in Western Europe, indicated that large differences in antigen content exist between these vaccines. A comparison of the antigen content with the serological response after vaccination with 1/50 vaccine dose demonstrated that a reliable estimate of the potency can be made, based on the HN-antigen content of the vaccine. We therefore present our antigen quantification assay for NDV as a candidate in vitro potency assay. An international feasibility and validation study will be needed to prove the suitability and reliability of this candidate in vitro potency assay for inactivated NDV vaccines.