An improved single-radial-immunodiffusion technique for the assay of influenza haemagglutinin antigen: Application for potency determinations of inactivated whole virus and subunit vaccines

An improved single-radial-diffusion technique for the assay of influenza haemagglutinin antigen is described. The modified method enables the results of assays of antigen to be obtained more rapidly and with greater precision than previously. The use of immunoplates containing varied, pre-selected concentrations of anti-haemagglutinin antibody allows accurate assays to be performed over a wide range of antigen concentrations. Concentrations of haemagglutinin as low as 40 i.u./ml could be assayed with accuracy and reproducibility using immunoplates containing low antibody levels. The method is applicable to the accurate determination of haemagglutinin concentrations over the ranges likely to be present in inactivated influenza vaccines. In tests on ‘whole virus’ antigen preparations, it was found that the ratio between haemagglutination titre (i.u./ml) and haemagglutinin antigen activity (μg/ml) determined by single-radial-diffusion was relatively constant for antigens containing a given strain but showed variation between strains (range 16·5–26·8 i.u./μg HA activity). For the subunit vaccines examined this ratio showed a large degree of variation (range 1·4–16·6 i.u./μg HA activity) and in general was considerably lower than for whole virus antigens. These findings support the conclusion that techniques based on the agglutination of erythrocytes may provide data on vaccine potency which are not directly comparable from strain to strain for ‘whole virus’ vaccines and that these methods are entirely inappropriate to potency assays of split-product or subunit vaccines. In contrast, single-radial-diffusion may be of value for assays of both ‘whole virus’ vaccines and those containing disrupted virions.