Tetanus antibodies measured by the toxin binding inhibition test (ToBI) in mothers and children in the Neonatal Tetanus Program in Vietnam.

The immunoresponse to vaccination in the Neonatal Tetanus Program (NNT) for pregnant women was studied in Vietnam using the Toxin Binding Inhibition Test (ToBI). The vaccination schedule consisted of two primary doses of adsorbed tetanus toxoid (TT) vaccine given with a one month interval. The seroconversion rate in the women was 98%. Two and a half months after birth, 63% of the children born from these women had tetanus antibody values higher than 0.01 IU/ml. Four women who had anti-tetanus titres < 0.01 IU/ml at delivery, despite two doses of primary vaccination, received a third booster with vaccine one year after the first injection. Their antibody levels were well above 0.01 IU/ml one month after this additional booster, suggesting that (when economically feasible) a third TT injection could be considered into the NNT to confer optimal anti-tetanus antibody levels in women for subsequent pregnancies. This study confirmed the effectiveness of the TT vaccines investigated and indicates their potential to replace, in immunosurveillance studies under field conditions, the in vivo mouse neutralisation test by in vitro alternative methods such as the ToBI test.

Validation of the combined toxin-binding inhibition test for determination of neutralizing antibodies against tetanus and diphtheria toxins in a vaccine field study in Viet Nam.

Determination of seroconversion and measurement of protective antibody levels in children against vaccine components are essential for gauging and monitoring the efficacy of paediatric vaccination programmes. For this purpose, we assessed the combined toxin-binding inhibition (ToBI) test for determining neutralizing antibodies to tetanus and diphtheria in a diphtheria-pertussis-tetanus (DPT) vaccine field trial in Viet Nam. A simple procedure involving collection of blood samples on filter-paper was found to be a suitable alternative to collection by venepuncture, despite a reduction in the sensitivity of the ToBI test as a result of the step necessary to elute the antibodies from the filter-paper. The results obtained demonstrate that the ToBI test can feasibly be carried out under field conditions. Preliminary results obtained with the ToBI test in DPT field trials indicate that a fourth dose of DPT vaccine one year after the third dose should be considered by developing countries.

PIP: In Vietnam, health workers collected blood samples from adults working at the National Institute of Vaccines and Biological Substances in Nha Trang and Dalat and from healthy unvaccinated infants 3-9 months old from the district areas/provinces of Tien Giang and Lam Dong to assess the combined toxin-binding inhibition (ToBI) test for determining neutralizing antibodies to tetanus and diphtheria in a diphtheria-pertussis-tetanus (DPT) vaccine field trial. Researchers also aimed to validate the use of a simple filter-paper blood collection method. There was a necessary step to elute the antibodies from the filter-paper, which reduced the sensitivity of the ToBI test. Nevertheless, in the ToBI test, blood collected on filter-paper yielded similar antibody titer estimations as those obtained by venepuncture. The Biotek method to estimate antibody titers yielded higher correlation coefficients than the OD50 method: tetanus (0.998 vs. 0.98) and diphtheria (0.956 vs. 0.95). One month after the third injection, all the children had antitoxin titers greater than 0.06IU/ml. By one year after the third injection, only 45% had antitoxin titers greater than 0.06IU/ml for diphtheria while all still had antitoxin titer levels above this value for tetanus. This suggests that health providers should consider administering a fourth dose of DPT vaccine one year after the third dose. These findings indicate that the ToBI test can be conducted under field conditions.