Persistence of antibody and response to booster dose of Haemophilus influenzae type b polysaccharide diphtheria toxoid conjugate vaccine in infants immunized at 9 to 15 months of age

Prevention of Haemophilus influenzae type b disease

Haemophilus influenzae type b (Hib) is the leading cause of meningitis in children < 5 years of age. The majority of cases of Hib occur in infants < 2 years of age. Until recently the only vaccine available against this disease contained the pure polysaccharide (PRP) of Hib (Hib-PRP vaccine). The Hib-PRP vaccine was demonstrated to be efficacious in infants > 18 months of age but not below that age. This product was licensed for routine use in the USA for children aged 24 months or more. Recently a hyperimmune globulin termed bacterial polysaccharide immune globulin (BPIg) was prepared by immunizing adult donors with Hib-PRP, meningococcal and pneumococcal vaccines. BPIg has been demonstrated to prevent Hib infections when it is administered to infants at 4-month intervals. However, BPIg has not been licensed for routine use in the USA. A number of new Hib conjugate vaccines have also been developed in the last few years by convalently linking the Hib-PRP to different carrier proteins. Four different Hib conjugate vaccines have undergone clinical trials in the USA. Two of these vaccines, HbOC (Hib capsular oligosaccharide linked to CRM197) and Hib-OMPC (Hib capsular polysaccharide linked to Neisseria meningitidis outer membrane protein complex) have been demonstrated to protect infants aged 2 months or more from Hib disease. Both HbOC and Hib-OMPC are currently licensed for routine use in the USA. The widespread use of these vaccines should have a substantial impact in reducing morbidity and mortality from Hib disease.

New vaccines against Haemophilus influenzae type b

Recently, great progress has been made in the development of vaccines against Hib. Four polysaccharide-protein conjugate vaccines are actively being tested. Three of these (PRP-D, HbOC, and PRP-OMP) are currently licensed for use in children at 15-18 months of age. Clinical trials of these vaccines in infants are currently being conducted in the United States. If these show the vaccines to be efficacious, licensure for infants will follow. Although much work remains to be done, it seems likely that the effective prevention of serious Hib infections in infants, as well as in older children, is a goal that may be within our reach in the next several years.

Bacterial vaccines for splenectomized patients

The spleen is an important organ in the defense of the body against pathogenic bacteria. Major functions of the spleen include antibody production and mechanical filtration of blood. Anatomically or functionally asplenic individuals are at increased risk of fulminant infection by encapsulated bacteria, particularly Streptococcus pneumoniae, Hemophilus influenzae, and Neisseria meningitidis. Polysaccharide vaccines are available against some strains of these pathogenic bacteria. More data are required to define specific age and risk groups. A search for better and more immunogenic vaccines, which may prove effective in a wider variety of patients, is currently under way. Although the current vaccines are not always effective and future revaccination may increase the incidence of adverse effects, most asplenic persons should receive the currently available vaccines to minimize their risk of life-threatening infection.

Efficacy of Haemophilus influenzae type b polysaccharide-diphtheria toxoid conjugate vaccine in infancy

Haemophilus influenzae type b capsular polysaccharide-diphtheria toxoid conjugate vaccine has recently been shown to be capable of inducing antibodies to H. influenzae in infants. In an evaluation of its clinical efficacy, 60,000 children were enrolled in an open trial in Finland. Children born on odd-numbered days between October 1, 1985, and September 30, 1986, received the vaccine at 3, 4, 6, and 14 months; those born on even-numbered days served as controls. The geometric mean antibody titer measured in a cohort of 99 children rose from a prevaccination level of 0.08 microgram per milliliter at three months of age to 0.42 microgram per milliliter at seven months. Only minor adverse reactions were reported. Up to February 1987, two cases of invasive H. influenzae infection had occurred among the children who had received three doses of vaccine, whereas 12 cases had occurred among the controls (P = 0.0005 by Poisson one-tailed test). The rate of short-term (average follow-up time, five months) protection provided by this conjugate vaccine in infancy was thus 83 percent.

Immunologic priming to capsular polysaccharide in infants immunized with Haemophilus influenzae type b polysaccharide-Neisseria meningitidis outer membrane protein conjugate vaccine

Thirty children vaccinated at 2 to 17 months of age with Haemophilus influenzae type b polysaccharide linked to a partially purified 40,000 dalton outer membrane protein of Neisseria meningitidis were revaccinated 10 to 14 months later with conventional H. influenzae type b polysaccharide vaccine. The geometric mean anti-type b antibody concentration before reimmunization was 0.68 micrograms/mL, and rose to 31 micrograms/mL in sera obtained 1 month later. The mean level after immunization was not significantly different than that in sera from 12 adults immunized with type b polysaccharide vaccine (51 micrograms/mL, P = 0.3), and was 10-fold higher than that of 13 control children immunized with type b polysaccharide for the first time (2.7 micrograms/mL, P less than 0.001). The IgG responses of the children first given conjugate vaccine and then conventional type b polysaccharide vaccine were of a similar magnitude as those in the immunized adults. Further, the children maintained high levels of serum antibody 6 to 8 months later. Ten other children vaccinated in infancy with conjugate vaccine, and again with conjugate vaccine 10 to 15 months later, showed similar antibody responses to those of the group given conjugate vaccine in infancy, and booster with conventional polysaccharide vaccine. Thus vaccination with H. influenzae type b polysaccharide-outer membrane protein conjugate vaccine primes the immune system to an IgG memory antibody response to either type b polysaccharide or conjugate vaccine. Post-booster sera from all children tested showed high titers of functional activity in a complement-mediated bactericidal assay. These data suggest that protection of most infants from type b Haemophilus disease may be achieved by a combination of immunization at 2 to 4 months of age with this conjugate vaccine, and reimmunization 1 year later with conjugate or conventional type b polysaccharide vaccine.

Safety and immunogenicity of Haemophilus influenzae type b polysaccharide and polysaccharide diphtheria toxoid conjugate vaccines in children 15 to 24 months of age

To evaluate the safety and immunogenicity of the Haemophilus influenzae type b polysaccharide vaccine, PRP, and a new polysaccharide-diphtheria toxoid conjugate vaccine, PRP-D, a collaborative study was carried out in six centers in five states. Subjects were 585 infants 15 to 24 months of age. They were randomly assigned to receive a single dose of PRP or PRP-D vaccine. There were no significant differences in the rate of adverse reactions between the two vaccine groups. Minor local reactions occurred in 10.3% of PRP and 12.5% of PRP-D recipients, and fever in 27.4% of PRP and 23.8% of PRP-D recipients. All reactions resolved within 48 hours. Serum samples were obtained just before vaccination and after 1 month. Prevaccination antibody levels were similar for the PRP (0.035 micrograms/mL) and PRP-D (0.027 micrograms/mL) groups, with no differences in levels by age, sex, race, vaccine lot, or study site. Both groups had significant rises in geometric mean levels, but this difference was significantly greater for PRP-D (2.166 micrograms/mL) than for PRP (0.154 micrograms/mL). In addition, the percentage of responders as determined by three definitions (twofold titer rise, greater than 0.15 micrograms/mL, and greater than 1.0 micrograms/mL) was also significantly greater for PRP-D than PRP. In contrast to a marked age-related immunogenicity to PRP (P less than 0.001), there was no significant variation in immune response to PRP-D by age. PRP-D conjugate vaccine appears to be as safe and significantly more immunogenic than PRP vaccine for children vaccinated at 15 to 24 months of age.