HAEMOPHILUS-INFLUENZAE TYPE-B CARRIAGE AND IMMUNITY 4 YEARS AFTER RECEIVING THE HAEMOPHILUS-INFLUENZAE OLIGOSACCHARIDE-CRM(197) (HBOC) CONJUGATE VACCINE

Late in 1991, before the implementation of a national immunization pro gram against Haemophilus influenzae type b (Hib) in the United Kingdom , we performed a 4-year follow-up of 120 children who in 1987 had been enrolled in an immunogenicity trial in which 60 of them (vaccinees) r eceived an Hib conjugate vaccine (HbOC) at the same time as diphtheria -tetanus toxoid-pertussis vaccine at the ages of 3, 5 and 9 months. Si xty others (controls) received only diphtheria-tetanus toxoid-pertussi s vaccine at the same ages and were not subsequently immunized against Hib. We investigated Hib pharyngeal colonization using the antiserum agar method and the concentrations of serum IgG antibody to the type b capsule by enzyme-linked immunosorbent assay. At 4 years of age the H ib colonization rates in vaccinees and controls were 8% (5 of 60) and 5% (3 of 60), respectively. The children colonized with Hib had greate r serum anti-capsular IgG concentrations than did noncolonized childre n (P < 0.001), and colonized vaccinees tended to have higher concentra tions than colonized controls (P = 0.053). Regardless of Hib colonizat ion status vaccinees had greater antibody concentrations than controls (P < 0.001). Forty-nine percent of vaccinees had an antibody concentr ation > 1 mug/ml. There was an inverse relationship between the Hib co lony count on culture and the serum IgG concentration. These data indi cate that the increase in serum antibody concentration after immunizat ion with an Hib conjugate vaccine is sustained and that immunization p rimes for a booster response on exposure to Hib. There may be a relati onship between previous Hib conjugate immunization and the density of Hib colonization in children.