Laboratory testing of whole cell pertussis vaccine: a WHO proficiency study using the Kendrick test.

Whole cell pertussis vaccine (WCV), commonly in combination with vaccines for diphtheria and tetanus, has an important role in reducing morbidity and mortality among children in most parts of the world. Testing to assure the efficacy of such vaccines is essential. We have, therefore, carried out, under the Global Training Network (GTN) of the Department of Vaccines and Biologicals at the World Health Organization (WHO), a proficiency study involving 13 laboratories in 12 countries that routinely test WCV. Two vaccine samples were tested in this study and represented samples which were expected clearly either to pass (sample B, a full strength vaccine) or to fail (sample A, 1/8 strength of vaccine B). Data from this study showed good performance by the majority of participants. Most assays were statistically valid and were carried out to the level of precision achieved for these assays in previous studies. This study also indicated that, relative to the assay precision, the in-house reference (IHR) preparations are in general accurately calibrated. Statistically valid assays of the sub-potent vaccine, A, showed it to fail in all except one laboratory. Statistically valid assays of the potent vaccine, B, showed it to pass in all laboratories. Nevertheless, the between laboratory variability of estimates for vaccine B, and for comparisons of the two vaccine samples suggested that there are some differences in results in different laboratories. The introduction of a common working standard may assist in reducing inter-laboratory variation. This study has shown clearly satisfactory performance by most laboratories. However, a serious problem was detected in one laboratory where the sub-potent vaccine A could have been passed and was not distinguished from the eight-fold more potent vaccine B. There were also indications of possible problems in several other laboratories, where IHR preparation may not be accurately calibrated or where vaccine samples A and B may not be completely distinguished. Although this study provides reassurance that most laboratories perform well, it demonstrates the essential role of ongoing proficiency studies in high-lighting problems.