Nguyen Thi My Thanh et al.
Laboratory of Enteric Bacteriology
We developed an ELISA procedure modified from the technique of the N.I.H. (U.S.A.) to determine Vi antibody concentration in serum. Purified Vi polysaccharide produced at the Vaccine Institute of Vietnam was used to coat 96 well-microtest plates. The sera were distributed in the wells of the plate, incubated overnight at room temperature. After washing, the conjugate (peroxidase-labelled goat antibody against human IgG) was distributed. The plate was incubated 4 hours at 37oC. After washing, the enzymatic activity development solution (O.P.D. in buffer with H2O2) was distributed into all wells. The reaction was allowed to develop in the dark for 30 minutes at room temperature, then stopped by 2M sulfuric acid. The optical densities were read at 492/620 nm using a microplate reader. The concentration of Vi antibody in each sample was calculated into ELISA Unit/ml or mg/ml based on the comparison with the standard antiserum. We used this technique to examine the immune response of 55 children of 5–14 years of age in Dong Thap province after the immunization with Vi conjugate vaccine. The Vi antibody concentrations in the pre-immune sera and the post-immune sera obtained 6 weeks after the vaccination were determined. All of them showed a significant increase in Vi antibody concentrations. 46 children (83.6%) had an increase of more than 100 times. The minimum of them was 17.8 times, the maximum was 2829 times.
This technique will be applied to evaluate the efficacy of the Vi vaccines produced in the country; to investigate the immunity response of different vaccinees; to investigate the normal levels of Vi antibody according to age, sex, residence…; to detect typhoid carriers and cure them in order to minimize the risks of transmission of the pathogen in the communities; and to evaluate the efficiency of old or new antibiotics in the systematic treatment of typhoid.