Single Radial Immunodiffusion (SRID)

The Single Radial Immunodiffusion (SRID) assay is a well‑established immunochemical method for the quantitative determination of hemagglutinin (HA) antigen content in influenza virus preparations. The assay is primarily applied to inactivated influenza A and B vaccines and is officially recognized by regulatory authorities, including the European Medicines Agency (EMA), as the reference method for influenza vaccine potency testing. SRID is based on the radial diffusion of HA antigens through an agarose gel containing strain‑specific anti‑HA antibodies, resulting in the formation of a visible antigen–antibody precipitin ring. Following incubation, the diameter of the precipitation zone is measured and compared against a calibrated reference antigen, allowing accurate quantification of HA content. Results are expressed as micrograms of HA per vaccine dose, with defined acceptance criteria supporting batch release and regulatory compliance.