Interpretation of responses and protective levels of antibody against attenuated influenza A viruses using single radial haemolysis.

Antibody determinations against H3N2 and H1N1 type A influenza viruses were carried out on paired sera obtained from volunteers taking part in influenza virus vaccine studies, using both the haemagglutination-inhibition (HI) and single radial haemolysis (SRH) test. Good correlation between the HI and SRH test was found for both H3N2 and H1N1 antibody and the zone area increases corresponding to significant SRH antibody rises determined for both virus strains. In both H3N2 and H1N1 vaccine studies, intranasal infection of the volunteers with live attenuated viruses was involved and by the measurement of HI and SRH antibodies prior to and following infection, levels of antibody equating with protection against the infecting viruses could be estimated. For the HI test the antibody titres associated with 50% protection were 42 for H1N1, and 44 for H3N2 viruses; for the SRH test, 50% protection was associated with zone areas of 20.0-25.0 mm2 for both H1N1 and H3N2 viruses.