Measurement of hemagglutination-inhibiting antibody to influenza virus in the 1976 influenza vaccine program: methods and test reproducibility.

Titers of hemagglutination-inhibiting (HAI) antibody were determined for all sera obtained from participants in the 1976 Influenza Vaccine Test Program. At least eight control sera were included in each test during the vaccine trial period for the purpose of monitoring HAI test reproducibility. Estimates of day-to-day reproducibility were defined as the percentages of duplicate aliquots of the same sera, tested on two separate days, having HAI antibody titers that did not differ by more than one twofold dilution. These reproducibility estimates ranged from 89% to 97% with influenza A/New Jersey/76 and A/Mayo Clinic/74 antigens. In contrast, within-day reproducibility estimates obtained from all sets of control sera ranged from 96% to 98%. Estimates of day-to-day test reproducibility obtained with selected sera taken after vaccination that were titrated on two differen days ranged from 90% to 98%. Geometric mean titers of these sera tested weeks or months apart differed on some occasions during the test period.