The use of the single radial haemolysis test for assessing antibody response and protective antibody levels in an influenza B vaccine study

Single radial haemolysis (SRH) and HAI tests were used to determine the levels of antibody in sera obtained from 118 volunteers taking part in a study of influenza B/Hong Kong/73 subunit vaccine. The two tests gave similar overall results, and showed good correlation, but SRH appeared to be more sensitive for the detection of low concentrations of antibody. A greater than or equal to 45% increase in SRH zone area was found to be equivalent to a significant increase in antibody titre and on the basis of this value, the HAI and SRH tests gave similar results for responses to immunization. The concentration of antibody equivalent to a 50% protective level against a challenge virus infection was a zone area of 45 mm2 for the SRH test; this was equivalent to an HAI titre of 1:20 which has previously been established as the 50% protective level.