SERUM ANTIBODIES IN SIX-YEAR-OLD CHILDREN VACCINATED IN INFANCY WITH A HAEMOPHILUS INFLUENZAE TYPE b-TETANUS TOXOID CONJUGATE VACCINE (1996)

Controlled trials with conjugate vaccines against Haemophilus influenzae type b (Hib) have demonstrated a high efficacy in preventing invasive Hib disease in children. The protection is mediated through vaccine-induced serum antibodies against the capsular polysaccharide (CPS) of Hib.1-3 It is not yet known for how long vaccine-induced serum antibodies remain above protective levels and how general Hib vaccination will affect the epidemiology of Hib disease in the long run. The aims of this study were to examine the persistence of serum antibodies against Hib at the age of 6 years in Swedish children who had been vaccinated with a Hib-tetanus toxoid conjugate vaccine (Hib-TT) at 3, 5 and 12 months of age and to compare Hib antibody concentrations in vaccinated and unvaccinated 6-year-old children. Subjects and methods. In 1987, 85 healthy, term, Swedish infants were vaccinated at 3, 5 and 12 months of age with Hib-TT, Lot NIH-45059, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, concurrently with the routinely used Swedish diphtheria-tetanus toxoid vaccine at separate sites. Forty children received 7.5 μg and 45 children received 15 μg of Hib CPS conjugated to tetanus toxoid. Safety data and antibody concentrations in sera obtained between 3 and 30 months of age have been reported.4, 5 All children responded to the vaccine. At 30 months of age geometric mean (GM) Hib antibody concentrations were 0.92 and 1.69 μg/ml in the 2 groups, 72 of 74 children (97%) had concentrations above the proposed protective level of 0.15 μg/ml1-3 and 38 of 74 children (51%) had antibodies above 1.0 μg/ml. In our study parents of 70 of the 85 original participants agreed to allow the child to donate serum at 6 years of age. None had experienced invasive Hib disease or any other serious disease. Sera from 85 healthy, Swedish 6-year-old children who were not vaccinated with a Hib vaccine and who had not experienced invasive Hib infection were used as controls. The sera from the control children were collected in connection with minor elective surgery. Sera were stored frozen at -20°C. Total amount of serum antibodies against Hib CPS were measured by radioimmunoassay6 at the University of Rochester, NY. The limit of antibody detection was 0.03 μg/ml. In the previous study antibodies were measured by radioimmunoassay at the National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, ME.4, 5 An unpaired t test was used to compare GM concentrations between groups. Proportions were compared with the corrected chi square test. The study was approved by the Ethics Research Committee of Göteborg University. Parents of vaccinated and control children gave oral consent after receiving oral and written information. Results. GM serum concentrations of Hib antibodies in the 70 vaccinees and 85 controls and the proportion of children with antibody ≥0.15 and ≥1.0 μg/ml, are shown in Table 1. All vaccinated and unvaccinated children had detectable Hib antibodies. GM concentration was significantly higher in children, who had received the higher dose of Hib-TT, than in controls (P < 0.05). No significant differences between high and low dose recipients or between low dose recipients and controls were observed. Of the vaccinees 97% had antibody concentrations ≥0.15 μg/ml compared with 87% of controls (P < 0.05). Sixty-six percent of the vaccine recipients and 47% of the controls had serum antibodies ≥1.0 μg/ml (P < 0.05). Discussion. Serum antibodies to Hib CPS, whether passively acquired by transplacental transmission or actively acquired with age, confer protection to systemic infection with this pathogen; almost all cases occur when placentally acquired antibodies have waned and before adult antibody concentrations are reached.7-10 In healthy individuals this age-related development of antibodies may occur after pharyngeal carriage of Hib or by a continual interaction with cross-reacting organisms.11-13 The latter probably accounts for most of this antigenic stimulus because the incidence of asymptomatic carriage of Hib is low (≅3%) in children and extremely rare in adults. Accordingly Hib antibody synthesis in adults is maintained in the absence of the homologous organism, and vaccination is designed primarily for inducing protective levels in infants and young children before naturally acquired antibodies have reached protective levels. The assumption of a protective serum value of approximately 0.15 μg of Hib antibody/ml, as measured by radioimmunoassay, has been based on the inverse correlation between the incidence of invasive Hib disease and antibody levels in nonvaccinated populations1, 2 and on a study of infants given hyper-immune globulin.3 A Hib conjugate vaccine should be immunogenic in infants and vaccine-induced serum antibodies should remain above protective levels until at least 6 years of age, when naturally acquired antibodies have reached the protective value in most children. Our study shows that the Hib-TT vaccine tested fulfilled this requirement for both doses used. The commercially available Hib-TT vaccine, Act-Hib® (Pasteur-Mérieux Sérums et Vaccins, Marcy L'Étoile, France), contains 10 μg of Hib CPS per dose. The Hib CPS is conjugated to tetanus toxoid in the same way as in the experimental Hib-TT used in this study. In the present study GM antibody concentrations were somewhat higher in vaccinated than in nonvaccinated children at 6 years of age, and significantly fewer vaccinated (3%) than unvaccinated (13%) children had serum antibodies below the protective level of 0.15 μg/ml.1-3 In contrast to children in many other countries, Swedish children receive only three doses of diphtheria, tetanus and injectable polio vaccine during the first year of life. This policy has also been adopted for the costly Hib conjugate vaccine. Future epidemiologic studies will show whether this three-dose regimen is sufficient to eradicate invasive Hib disease. The immunologic data presented here indicate that this should be possible. Acknowledgments. We are grateful to Nina Knutsson and Susan Porcelli for skillful technical assistance. The study was supported by the Faculty of Medicine, Göteborg University, Sweden and by Contract NOI AI 45196, National Institutes of Health, Bethesda, MD. Bo A. Claesson, M.D.; Birger Trollfors, M.D.; Porter W. Anderson, M.D.; Jan Johansson, M.D.; John Taranger, M.D.; Rachel Schneerson, M.D.; John B. Robbins, M.D Departments of Infectious Diseases (BAC) and Pediatrics (BT, JT) Göteborg University Göteborg, Sweden Department of Pediatrics Borås Hospital Borås, Sweden (JJ) Department of Pediatrics University of Rochester Rochester, NY (PWA) Laboratory of Developmental and Molecular Immunity National Institute of Child Health and Human Development National Institutes of Health Bethesda, MD (RS, JBR)