Application of an improved single-radial-immunodiffusion technique for the assay of haemagglutinin antigen content of whole virus and subunit influenza 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, preselected concentrations of antihaemagglutinin antibody allows accurate assays to be performed over a wide range of antigen concentration. In tests on "whole virus" antigen preparations it was found that the ratio between haemagglutination titre (iu/ml) and haemagglutinin antigen activity (microgram/ml) determined by single-radial-diffusion was relatively constant for antigens containing a given strain but showed variation between strains (range 16.5 to 26.8 iu/microgram HA activity). For the subunit vaccines examined this ratio showed a large degree of variation (range 1.4 to 16.6 iu/microgram HA activity) and in general was considerably lower than for whole virus antigens. It was concluded that single-radial-diffusion may be of value for assays of the haemagglutinin concentration of both "whole virus" vaccines and subunit vaccines. The data presented suggest that, in the event of antigenic drift, it may be possible to use the existing reference anti-haemagglutinin serum to assay a variant strain of influenza virus.