Single-radial-immunodiffusion as an in vitro potency assay for human inactivated viral vaccines

Single-radial-immunodiffusion (SRID) assays have been used to determine the potency of all human inactivated influenza virus vaccines licensed by the Food and Drug Administration for use in the United States since 1978. SRID replaced less reliable tests which were based on the aggregation of erythrocytes by the hemagglutinins of influenza viruses. Similar SRID assays have been used experimentally to determine the potency of inactivated polio and rabies vaccines. In each case, the assays are based on the diffusion of viral antigen into an agarose gel containing specific antibodies to the antigen being measured. For influenza and rabies, disruption of the virions with a detergent is necessary to permit the diffusion of the appropriate antigens, where as with polio, intact virions are allowed to diffuse. The interaction between antigen and antibody produces a zone of precipitation whose size is directly proportional to the amount of antigen applied. A potency value for unknowns is obtained by comparing the sizes of zones produced by unknown preparations to the sizes of zones obtained with a calibrated reference of known antigen content. Once the specific reference antigens and antibodies are prepared and the test standardized, it is a remarkably simple technique which unlike agglutination assays is very reproducible, relatively unaffected by minor variations in test conditions and is far less time consuming and cumbersome than in vivo assays for potency such as those done by inoculating mice or monkeys. More importantly, clinical studies demonstrate that standardization of influenza vaccines by SRID provides a better correlate of human immunogenicity than previous methods.