Immunogenicity of Hib-TT conjugate vaccine in 6-59 months-old young children and the antibody persistence in 3-5 months-old infants after primary and boosting injection with Hib-TT and Act-Hib conjugate vaccines

Objective To evaluate the immunogenicity of a Haemophilus influenzae type b capsular-tetanus toxoid(Hib-TT) conjugate vaccine produced by Lanzhou Institute of Biological products(LIBP).Methods In an open-controlled,randomized trial,the eligible and consented 6-59 months-old young children injected 2 or 1 times 1 month apart with Hib-TT conjugate vaccine,the 3-5 months-old infants received 3 injections 1 month apart for primary immunization with Hib-TT or a licensed international Hib-TT conjugate vaccine as the control vaccine,and the boosting dose of two 3-5 months-old groups was injected at the 15-17months-old.The serum anti-Hib PRP IgG GMC in both groups after primary and boosting vaccination was measured by ELISA,the percentage of geometric mean concentration (GMC) ≥ 0.15 μg/ml and ≥ 1.0μg/ml was calculated,respectively.Results The Hib-TT conjugate vaccine produced in LIBP elicited satisfactory IgG antibody response in 3-59 months-old young children,the serum IgG GMC of anti-Hib PRP were 14.52 μg/ml(95％ CI:12.31-17.14)in 3-5 months-old,14.04 μg/ml(95％ CI:12.40-15.90) in 6-11 months-old.the ratios of IgG antibody concentration ≥ 1.0 μg/ml were 96.90％ (95％ CI:92.50-99.20) in study vaccine group and 98.55％ (95％ CI:92.20-99.90) in the control vaccine after 3 doses,respectively.100％ of the 6-11 months-old young children who injected 2 times with the Hib-TT conjugate vaccine had IgG antibody concentration ≥ 1.0 μg/ml (95％ CI:95.94-100.00),91.35％ (95％ CI:86.13-99.48) of recipients in 12-59 months-old young children induced the IgG antibody concentration ≥ 1.0 μg/ml after a single dose.The serum IgG antibody GMC in recipients who received the study or and control vaccines increased from 6.27 μg/ml (95 ％ CI:5.28-7.48) and 5.57 μg/ml (95 ％ CI:4.45-6.97)at pre-boosting injections to 63.14 μg/ml(95％ CI:52.14-76.47) and 73.48 μg/ml (95％ CI:57.37-94.11) one month after boosting injection,respectively.The percentage of IgG antibody concentration ≥ 1.0 μg/ml increased from 76.35％ and 79.55％ of pre-boosting to 100％ in the two groups after booting dose.Although the serum IgG GMC in two groups appeared to decline markedly,it remained at a relatively high levels of 25.02 μg/ml (95％ CI:20.51-30.48) in the study vaccine and 23.64 μg/ml (95％ CI:18.40-30.43) in the control vaccine,and all of the recipients in both groups remained 100.0％ of IgG antibody concentration ≥ 1.0 μg/ml.Conclusion The study vaccine elicited a protective immune response and induced the IgG antibody concentration which indicated long-term protection of anti-Hib PRP in 3 to 59months-old infants and young children. Key words: Hib-TT conjugate vaccine;  Immunogenicity;  GMC;  Primary immunization;  Boosting immunization