Passive hemagglutination (HA), a bactericidal activity test (BCA), and radioimmunoassay (RIA) were compared in measuring serum antibodies before and after group A meningococcal capsular polysaccharide vaccination of servicemen. The three methods were found satisfactory in demonstrating a response to vaccination in this age group. Of the postimmunization sera, 5% remained without HA and 1% remained without BCA activity; 1% of the postimmunization sera had less than 2 micrograms of antibody per ml as measured by RIA. Approximately 60% of the serum pairs showed a greater than or equal to 32-fold rise in HA titer, a greater than or equal 25-fold rise in BCA titer, or a greater than or equal to 4-fold rise in antibody concentration by RIA. A difference in response to two different vaccine lots was seen with RIA and BCA. Although the calculated correlation between the three methods was good, some individual sera gave discrepant results. These could be shown to be due mainly to one of the following factors: low HA titer was due to lack of the immunoglobulin M and A classes of antibodies, low BCA titer was due to the blocking effect of high immunoglobulin A content, and high BCA titer was due to antibodies directed to bacterial components other than the capsular polysaccharide.
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