Lack of comparability between commonly used serological assays of immune response to Haemophilus influenzae vaccine.

Reliable measures of the serum level of antibody to the capsular polysaccharide (PRP) of Haemophilus influenzae type b (Hib) are essential for evaluating current and future vaccines intended to protect against invasive disease. It was recently noted, however, that certain commonly used assays of antibody to PRP produced incompatible results. To investigate this observation, we analyzed banked sera from four PRP vaccine studies in parallel by several assays. Significant differences were noted, both in absolute titers and in the ratios of post- to prevaccination titers, between the various assays. These differences seemed to be due to the different antigen preparations used in the assays. Unless workers standardize on a single assay, protective levels of antibody to PRP will have to be defined separately for each assay. Serological results of Hib vaccine trials cannot be compared without considering the assay methods used.