In Poland, similar to many highly immunized Western countries, a recent increase in cases of
pertussis has been observed. This study aims to evaluate the level of potency fluctuations of the
pertussis component of Polish-produced
DTP vaccine due to the changes having occurred in production and potency testing procedures. We compared the potency of the
pertussis component of
DTP vaccine lots produced and evaluated in similar periods and with similar production and testing procedures. Records of
Kendrick test results performed over a 30-year period were available for analysis. This study confirms the role of different manufacturers, changes in
vaccine strain compositions, in-house reference preparations used as reference
vaccines in the
Kendrick tests, and in mice of single strain sources in the potency values obtained. In addition, the comparisons performed revealed a downtrend in potency levels since 1992. Potency decrease in
vaccine lots produced during 1992-1997 has been positively correlated to the lowering of the number of IOU/dose. Strain compositions of the
DTP vaccine pertussis component and in-house references have been found to be associated with the fluctuation in potency estimations, and confirmed their crucial role in ensuring
vaccine efficacy. Our study reveals that relative efficacy of the
DTP vaccine produced in 1992-1997 might be lower than that of
vaccines produced in other periods. This might in turn explain the increase in
pertussis cases among children aged 5-15 years which is presently being observed in Poland.