Diagnostic pertussis serology in the recent clinical efficacy studies of acellular vaccines

The laboratory routine used and the criteria applied for serological case confirmation in vaccine efficacy trials have a direct influence on the identification of cases, which consequently may also affect the estimation of vaccine efficacy (VE). Some differences in the application of serological confirmation criteria among the recent clinical studies of pertussis vaccines include the level of increase in titre and use of single specimen diagnostics. Additionally, the use of pre-exposure serum specimen collections increases the sensitivity of serological confirmation. In the 1992-95 Stockholm trial, a regimen to collect serum samples systematically was introduced; using acute- and convalescent-phase sera from the cough episodes, the proportion of all cases which were serologically confirmed was 25%. When pre-exposure sera were also available, the proportion was 35%; the increased sensitivity was differential by vaccine group and affected the estimated VE to some extent. Therefore, with the different application of serological methods among the various efficacy studies, direct comparisons between studies should be made with great caution.