Standardization and control of toxoid components in the combined vaccine (DPT).

The previous paper (Yamamoto et al., 1972) stated that the potency of tetanus toxoid in the combined vaccine (DPT) varied greatly when assayed against the current National Standard Tetanus Toxoid (plain, NSTT) and that the latter might not be suitable as the reference for the assay of the toxoid component in combined vaccines Since the fact seemed to raise a serious problem about the validity of the current assay method (called MR64 method), another experiment was conducted to confirm the previous finding by using NSTT and two combined vaccines, DPT 368 and DPT 168/ 268SP. The first two were used in the previous experiment. The last one was a freeze-dried combined vaccine deprived of pertussis cells by centrifugation and added with normal guinea pig serum at 20 % to facilitate freeze-drying. On reconstitution with saline, the sample contained 50 Lf and 9 Lf of diphtheria and tetanus toxoids, respectively. The results are shown in Table I. Summarized data of the former experiments are shown in Table II for comparison. These results show clearly the inadequacy of the MR64 method for the potency test of tetanus toxoid in combined vaccine. The fact may explain why discrepancies were often observed in duplicate tests by National Institute of Health (NIH) and manufacturers in Japan (Yamamoto et al., 1972). Such discrepancies may not be rare in the bioassay of biological products (Hardegree, Pittman and Maloney, 1972; Mussett and Sheffield, 1973). On the other hand, the need for the accurate potency test of the biological products important for public health increased recently to avoid the overuse of the vaccine. Therefore, improvement of the assay method to minimize such discrepancies is urgently required for production and control of the combined vaccine DPT. A clue to improve the method is suggested in Tables I and II. It is evident that the relative potencies to a combined vaccine were less variable than those determined against NSTT (plain). Therefore, DPT 368 was tentativelyselected as the reference for potency tests of the toxoid components of other combined vaccines, since it had been tested repeatedly in our laboratory and used in the field trial carried out by the Research Committee on Pertussis and Combined Vaccine (Someya et al., 1972). However, difficulty was encountered to assign unitage of the tatanus component of the vaccine, when we tried to correlate it somehow to the current standard preparation (plain), as stated above. We finally decided to assign 200 protective units (PU) to the tetanus component of DPT 368, the value calculated relatively to the combined vaccine (called DPT RI68 in Table II) which had been used in the extensive collaborative study and had a potency of 200 IU/ml (van Ramshorst, Sundaresan and Outschoorn, 1972). This seemed reasonable since the variation in the relative potency of DPT 368 to that of DPT R168 was not significant (Table II). On the other hand, a value of 43.2 IU/ml was obtained