Philip P. Vella

The Efficacy in Navajo Infants of a Conjugate Vaccine Consisting of Haemophilus influenzae Type b Polysaccharide and Neisseria meningitidis Outer-Membrane Protein Complex

BACKGROUND AND METHODS Several conjugate vaccines against Haemophilus influenzae type b have been developed in the search for one that induces protection even in young infants. We evaluated the safety and efficacy of a conjugate vaccine that links the H. influenzae type b capsular polysaccharide to the outer-membrane protein complex (OMPC) of Neisseria meningitidis serogroup B. We conducted a double-blind, placebo, controlled trial in Navajo infants, who are at high risk for systemic infections caused by H. influenzae type b. The infants were randomly assigned to receive the first dose of vaccine or placebo at 42 to 90 days of age and the second at 70 to 146 days of age. RESULTS Of the infants in the trial, 2588 were assigned to receive the vaccine and 2602 to receive placebo. The mean follow-up was 269 days in the vaccine group and 267 days in the placebo group. Before the age of 18 months, there was 1 systemic H. influenzae type b infection in the vaccine group, as compared with 22 in the placebo group (P less than 0.001; point estimate of efficacy, 95 percent; 95 percent confidence interval, 72 to 99 percent). Of the 22 H. influenzae type b infections in the placebo group, 13 were meningitis. Among the children who received both doses, there was 1 H. influenzae type b infection in the vaccine group (n = 2056) and 14 in the placebo group (n = 2105) (P less than 0.001; point estimate of efficacy, 93 percent; 95 percent confidence interval, 53 to 98 percent). The single infection in the vaccine group occurred at 15 1/2 months of age in an infant with osteomyelitis. Between the first and second doses there were no H. influenzae type b infections in the vaccine group and eight in the placebo group (P less than 0.005; point estimate of efficacy, 100 percent; 95 percent confidence interval, 41 to 100 percent). CONCLUSIONS The H. influenzae type b OMPC vaccine, administered at 2 and 4 months of age, is safe and induces a high rate of protection against invasive disease caused by H. influenzae type b in infants under the age of 18 months. Protection begins after the first dose.

Safety and immunogenicity of Haemophilus influenzae type B-Neisseria meningitidis group B outer membrane protein complex conjugate vaccine mixed in the syringe with diphtheria-tetanus-pertussis vaccine in young Gambian infants.

To ensure compliance and to reduce costs it is important, especially in less developed countries, that programs of child immunization should require as few clinic attendances and as few injections as possible. Therefore we have investigated whether a Haemophilus influenzae type b conjugate vaccine could be given safely and effectively with diphtheria-tetanus-pertussis vaccine (DTP). One hundred twenty-six Gambian infants were given both polyribosylribitol phosphate (PRP)-outer membrane protein complex (PedvaxHIB) and DTP on the same day at 8, 12 and 16 weeks of age; 60 were given the vaccines mixed in the syringe and 66 were given the vaccines separately. To minimize the injection volume the dose of PRP-OMPC used in both groups was 7.5 micrograms, which is half the usual dose. There were no significant differences in anti-PRP antibody titers between the groups after 1, 2 or 3 doses. The geometric mean titers of antibody for the two groups combined were 0.29 micrograms/ml 1 month after the first dose, 1.03 micrograms/ml after the second dose and 1.11 micrograms/ml after the third dose. Concentrations of antibodies to diphtheria, tetanus and pertussis 1 month after the third dose were not significantly different between the two groups. Systemic side effects were reported with equal frequency in the two groups and were similar to those reported elsewhere for DTP. Tenderness at the injection site was more common where the combined injection (0.75 ml) had been given than where DTP alone (0.5 ml) had been given. The main drawback to the use of these 2 vaccines together is the complexity of the mixing procedure used in this clinical trial.

Immunogenicity of Haemophilus Influenzae Type b Conjugate Vaccines in Infant Rhesus Monkeys

ABSTRACT: Two Haemophilus influenzae type b (Hib) polysaccharide-protein conjugate vaccines were evaluated for immunogenicity in eliciting anti-polyribosyl ribitol phosphate (PRP) antibodies in infant rhesus monkeys. Animals received intramuscular injections of either Hib polysaccharide (PRP)-meningococcal outer membrane protein complex or Hib oliosaccharide-CRM197 (HbOC) conjugate vaccines on d 0,28, and 56. Because HbOC contains the CRM197 mutant diphtheria toxin from Corynbacterium diphtheriae as its protein carrier, the effect of simultaneous injection of diphtheria toxoid on the immunogenicity of HbOC also was evaluated by dividing monkeys vaccinated with HbOC into three groups: HbOC/saline, HbOC/diph-theria and tetanus toxoids, and HbOC/tetanus toxoid (coadministration of HbOC and other vaccine or placebo injected into the flank muscle of different legs). Infant monkeys vaccinated with the PRP-outer membrane protein complex conjugate responded with anti-PRP antibody after the first dose and showed booster responses after the second and third injections. In contrast, infant monkeys vaccinated with HbOC did not respond after three doses of HbOC/saline or HbOC/tetanus toxoid. However, two of three monkeys given concurrent injections of HbOC and diphtheria and tetanus toxoids did respond. The nonresponder monkey to three doses of HbOC and diphtheria and tetanus toxoids did respond to a subsequent injection with PRP-outer membrane protein complex. Thus, concomitant administration of diphtheria toxoid, a common vaccine for human infants, is necessary to elicit an anti-PRP antibody response to HbOC. This finding suggests that the CRM197 protein, at the 12.5-μg dose of protein in the HbOC used in this study, may not be sufficiently immunogenic in naive infant rhesus monkeys to permit an antibody response to the Hib oligosaccharide component of the conjugate.

Haemophilus b conjugate vaccines.

Publisher Summary This chapter discusses Haemophilus b conjugate vaccines. Haemophilus influenzae type b (Hib), an encapsulated, Gram-negative, rod-like bacterium, is responsible for an estimated 10,000 cases of meningitis per year in the United States. Other invasive Hib infections, such as septicemia, epiglottitis, cellulitis, septic arthritis, pneumonia, otitis media, osteomyelitis, and pericarditis can also occur, bringing the total number of invasive Hib infections in the United States to at least 20,000 cases annually. There is considerable evidence that antibodies to the Hib capsular polysaccharide are protective. The biological properties of these antibodies include complement activation for opsonic and bactericidal activity, and protection of infant rats from bacteremia and subsequent meningitis following challenge with virulent Hib organisms. In 1985, the first Hib vaccine, composed of purified PRP, was licensed in the United States.

Persistence of antibody at 18 months following vaccination of young Gambian infants with PRP-OMPC Haemophilus influenzae type b conjugate vaccine

The rate of decline in anti-PRP antibody levels was measured in two groups of Gambian children who had been given PRP-OMPC at 1 and 3 months or 2 and 4 months of age. In the younger group (n = 70), the geometric mean titre fell from 1.32 micrograms/ml at 4 months to 0.44 micrograms/ml at 18 months. In the older group (n = 54), the geometric mean titre fell from 1.18 micrograms/ml at 5 months to 0.46 micrograms/ml at 18 months. The proportion of vaccinated children with antibody levels over 1.0 microgram/ml fell from 54% 1 month after the second dose of vaccine to 27% at the age of 18 months, while the proportion with levels over 0.15 micrograms/ml fell from 82% to 60%, with no significant differences observed between the vaccination groups. For those children who did not show evidence of environmental boosting, the half-life of anti-PRP antibody was about 100 days. This did not differ between the groups. These findings suggest that to provide lasting immunity PRP-OMPC should be given with a late booster dose at 12-15 months, as is the current practice in the USA. The need for a late booster dose may limit the value of this vaccine in developing countries where vaccination of children is difficult after the 1st year of life.

Biological activity of Hib conjugates

Haemophilus influenzae type b (Hib) conjugate vaccines dramatically improve the immunogenicity against the capsular polysaccharide (PRP) of Hib. A new Hib conjugate, PedvaxHIB, is shown to be immunogenic in infant rhesus monkeys. The monkey model appears to correlate well with the immunogenicity of PedvaxHIB in human clinical studies. Not all commercial Hib conjugates are immunogenic in the monkey model. The data from the priming study indicate that HibTITER is not immunogenic in an immune system naive to diphtheria toxoid, such as the infant rhesus monkey. The role of diphtheria toxoid in the immunogenicity of HibTITER in human infants should be studied.

An enzyme-linked immunosorbent assay for quantitation of Haemophilus Influenzae type b polysaccharide-specific IgG1 and IgG2 in human and infant rhesus monkey sera.

An enzyme-linked immunosorbent assay (ELISA) has been developed and validated to quantitate IgG1 and IgG2 antibody to polyribosyl-ribitol phosphate (PRP), the capsular polysaccharide of Haemophilus influenzae type b (Hib). The sera of children and infant Rhesus monkeys immunized with an Hib conjugate vaccine composed of Hib PRP covalently linked to an outer membrane protein complex (OMPC) from Neisseria meningitidis serogroup B (PedvaxHIB, PRP-OMPC, Merck, Sharp and Dohme Research Laboratories). The solid-phase antigen employed in the ELISA is a conjugate of PRP to human serum albumin. The enzyme-labeled antibody is alkaline phosphatase-conjugated mouse monoclonal (mAb) anti-human IgG1 or IgG2. A human serum standard was calibrated using parallel titrations with a known antibody standard. The geometric mean titer (GMT) of the anti-PRP IgG1 response to one dose of PedvaxHIB was 3.87 micrograms/ml (n = 82), 11.80 micrograms/ml (n = 62) and 14.57 micrograms/ml (n = 74) in infants and children 12 to 17 months, 18 to 23 months and greater than or equal to 24 months old, respectively. Infants 2 to 11 months old responded with an IgG1 anti-PRP response of 7.10 micrograms/ml while infant monkeys responded with a GMT of 150.65 (n = 9) after two doses of vaccine. The anti-PRP IgG2 GMT responses in all groups were less than 0.25 micrograms/ml, except for humans greater than or equal to 18-months old who exhibited a GMT of greater than or equal to 0.40 micrograms/ml (n = 75). PedvaxHIB, immunization of human infants and children and infant Rhesus monkeys elicits primarily an IgG1 response to PRP. The monkey model appears to be a reliable indicator of the human immune response.