Safety, immunogenicity, and efficacy in infancy of oligosaccharide conjugate Haemophilus influenzae type b vaccine in a United States population: possible implications for optimal use

The CAPITA study of protein-conjugate pneumococcal vaccine and its implications for use in adults in developed countries

Until 1990, Hemophilus influenzae type b (HITB) was a major cause of morbidity and mortality in toddlers and young children. A vaccine consisting of purified polyribosyl ribitol phosphate (PRP), the capsular polysaccharide (CPS) of HITB, had been shown to be ineffective as an antigen in the population at risk, and this vaccine was withdrawn from the market within a few years of its introduction. By contrast, the discovery that PRP, when covalently bound to an antigenic protein, stimulated antibody production in infants and toddlers, (1) led to the development of a vaccine that has all but eradicated HITB infection and brought about a near-disappearance of this organism in the United States.

Positively selected codons in immune-exposed loops of the vaccine candidate OMP-P1 of Haemophilus influenzae

The high levels of variation in surface epitopes can be considered as an evolutionary hallmark of immune selection. New computational tools enable analysis of this variation by identifying codons that exhibit high rates of amino acid changes relative to the synonymous substitution rate. In the outer membrane protein P1 of Haemophilus influenzae, a vaccine candidate for nontypeable strains, we identified four codons with this attribute in domains that did not correspond to known or assumed B- and T-cell epitopes of OMP-P1. These codons flank hypervariable domains and do not appear to be false positives as judged from parsimony and maximum likelihood analyses. Some closely spaced positively selected codons have been previously considered part of a transmembrane domain, which would render this region unsuited for inclusion in a vaccine. Secondary structure analysis, three-dimensional structural database searches, and homology modeling using FadL of E. coli as a structural homologue, however, revealed that all positively selected codons are located in or near extracellular looping domains. The spacing and level of diversity of these positively selected and exposed codons in OMP-P1 suggest that vaccine targets based on these and conserved flanking residues may provide broad coverage in H. influenzae.

Prospects for vaccine prevention of meningococcal infection

Neisseria meningitidis is the leading cause of bacterial meningitis in the United States and worldwide. A serogroup A/C/W-135/Y polysaccharide meningococcal vaccine has been licensed in the United States since 1981 but has not been used universally outside of the military. On 14 January 2005, a polysaccharide conjugate vaccine that covers meningococcal serogroups A, C, W-135, and Y was licensed in the United States for 11- to 55-year-olds and is now recommended for the routine immunization of adolescents and other high-risk groups. This review covers the changing epidemiology of meningococcal disease in the United States, issues related to vaccine prevention, and recommendations on the use of the new vaccine.

Immunogenicity and efficacy of childhood vaccines in HIV-1-infected children

Children infected by HIV-1 are particularly vulnerable to severe, recurrent, or unusual infections by vaccine-preventable pathogens. Routine immunisations seem to be generally safe for HIV-1-infected children, but responses may be suboptimal. Early HIV-1-induced immune attrition associated with viral replication may particularly interfere with the development of memory responses. In high HIV-1 prevalence regions, the accumulation of susceptible hosts may compromise disease-control efforts. Although early control of viral replication through treatment with highly active therapy may preserve immune function and responses to routine childhood vaccines, availability is limited in the areas most affected. In this review, we provide an overview of the immunogenicity and efficacy of childhood vaccines in HIV-1-infected children. The possible immunological bases for defective responses are discussed; unanswered questions and the need for further research are delineated.

Vaccines and immunotherapies for the prevention of infectious diseases having cutaneous manifestations

Although the development of antimicrobial drugs has advanced rapidly in the past several years, such agents act against only certain groups of microbes and are associated with increasing rates of resistance. These limitations of treatment force physicians to continue to rely on prevention, which is more effective and cost-effective than therapy. From the use of the smallpox vaccine by Jenner in the 1700s to the current concerns about biologic warfare, the technology for vaccine development has seen numerous advances. The currently available vaccines for viral illnesses include Dryvax for smallpox; the combination measles, mumps, and rubella vaccine; inactivated vaccine for hepatitis A; plasma-derived vaccine for hepatitis B; and the live attenuated Oka strain vaccine for varicella zoster. Vaccines available against bacterial illnesses include those for anthrax, Haemophilus influenzae, and Neisseria meningitidis. Currently in development for both prophylactic and therapeutic purposes are vaccines for HIV, herpes simplex virus, and human papillomavirus. Other vaccines being investigated for prevention are those for cytomegalovirus, respiratory syncytial virus, parainfluenza virus, hepatitis C, and dengue fever, among many others. Fungal and protozoan diseases are also subjects of vaccine research. Among immunoglobulins approved for prophylactic and therapeutic use are those against cytomegalovirus, hepatitis A and B, measles, rabies, and tetanus. With this progress, it is hoped that effective vaccines soon will be developed for many more infectious diseases with cutaneous manifestations.

Changes in treatment strategies for acute otitis media after full implementation of the pneumococcal seven valent conjugate vaccine

BACKGROUND

Administration of the pneumococcal seven valent conjugate vaccine (PCV-7) results in significant reductions in the incidence of invasive pneumococcal disease and nasopharyngeal colonization rates; however, the reduction in the overall incidence of acute otitis media (AOM) is minimal. Despite an ongoing shortage of PCV-7 in the United States, during which children are receiving fewer doses at longer intervals than the recommended dosing schedule, the incidence of invasive pneumococcal disease in young children has continued to decrease.

PURPOSE

The purpose of this article is to review why the reported effect of PCV-7 on AOM infection rates does not match the reduction in invasive pneumococcal disease or the reduction in nasopharyngeal carriage of Streptococcus pneumoniae.

METHODS

The potential effect of a fully implemented PCV-7 schedule on AOM infections is calculated based on serotype-specific AOM reduction rates observed in clinical studies. Clinical success rates of various antimicrobials used in the treatment of AOM in children fully immunized with PCV-7 also are calculated based on projected PCV-7-induced changes in pathogen profiles and documented antimicrobial resistance rates. Questions regarding potential serotype substitution in children with AOM but not invasive pneumococcal disease, the increased incidence of infection with Haemophilus influenzae in children immunized with PCV-7 and the possibility of herd immunity also will be addressed.

RESULTS

Although the majority of penicillin-nonsusceptible S. pneumoniae are included in PCV-7, AOM caused by these strains will not completely disappear, especially in children 24 months of age and younger.

CONCLUSIONS

Consequently, clinicians must continue to consider penicillin-nonsusceptible pneumococci when prescribing antimicrobials for AOM.

Serological criteria for evaluation and licensure of new pneumococcal conjugate vaccine formulations for use in infants

The World Health Organization (WHO) is undertaking a series of consultations on serological criteria for the evaluation and licensure of new formulations/combinations or different vaccination schedules of pneumococcal conjugate vaccines. The lack of a definitive serological correlate of protection and the multiplicity of antigens involved, especially since the clinical efficacy of most of the individual serotypes represented in the only licensed vaccine has not been established, are hindering the formulation of criteria for licensure of new formulations or combinations of the vaccine. This report analyses the various options with their relative merits and drawbacks and provides preliminary recommendations as guidance to regulatory agencies in evaluating these vaccines for the purposes of licensure. More detailed recommendations for production and control of pneumococcal conjugate vaccines, including criteria for evaluation for licensure, are currently being drafted.

Effects of an efflux mechanism and ribosomal mutations on macrolide susceptibility of Haemophilus influenzae clinical isolates

This study investigated macrolide resistance mechanisms in clinical Haemophilus influenzae strains with different levels of susceptibility to macrolides. A total of 6,382 isolates were collected during the Alexander Project from 1997 to 2000. For 96.9% of these isolates, the azithromycin MICs were 0.25 to 4 micro g/ml, and these were defined as baseline strains. For 1.8% of the isolates, the azithromycin MICs were lower (<0.25 micro g/ml), and for 1.3% of the isolates, the MICs were higher (>4 micro g/ml). These isolates were defined as hypersusceptible and high-level macrolide-resistant strains, respectively. To identify the mechanisms associated with these three susceptibility patterns, representative strains were studied for the presence of macrolide efflux pumps and for ribosomal alterations. Macrolide efflux was studied by measuring the accumulation of radioactive azithromycin and clarithromycin in the presence or absence of carbonyl cyanide m-chlorophenylhydrazone (CCCP), a protonophore. Treatment with CCCP increased the accumulation of macrolides in baseline as well as high-level resistant strains, demonstrating the presence of an efflux mechanism, but not in the 20 hypersusceptible strains tested. Among the 31 strains studied that showed high-level resistance to both azithromycin and clarithromycin, 28 had ribosomal alterations, 7 had mutations in ribosomal protein L4, 11 had mutations in L22, 2 had mutations in 23S rRNA, 8 had multiple mutations, and 3 had no mutations. From these results, we conclude that the vast majority (>98%) of H. influenzae strains have a macrolide efflux mechanism, with a few of these being hyperresistant (1.3%) due to one or several ribosomal mutations. Occasional hypersusceptible strains (1.8%) were found and had no macrolide resistance mechanisms and appeared to be the only truly macrolide-susceptible variants of H. influenzae.

Assessing efficacy of Haemophilus influenzae type b combination vaccines

Regulatory approval of diphtheria and tetanus toxoids and acellular pertussis (DTaP)-based combination vaccines containing Haemophilus influenzae type b (Hib) has been delayed in the US because of difficulty in assessing the effect of lower Hib immunogenicity on vaccine efficacy compared with the immunogenicity of the specific Hib component administered separately. Hib conjugate vaccines confer protection by eliciting serum anticapsular antibody and priming for immunologic memory. Therefore, compelling proof of efficacy would be demonstration that a combination vaccine primes for memory and elicits antibody responses that are not inferior to those elicited by other US-licensed Hib conjugate vaccines, not necessarily the specific Hib component used in the combination. Vaccinated infants also can be considered protected if their serum anticapsular antibody concentrations are > or =0.15 microg/mL immediately before the booster dose given in the second year of life, when antibody concentrations are lowest. These alternative serologic approaches offer a strong scientific and regulatory rationale for licensure of effective DTaP-based Hib combination vaccines.

A review of the design of vaccine efficacy trials and a proposal for the design of the VA Cooperative Study of Active Immunotherapy of HIV Infection

We review the design of vaccine trials based on a search of the medical literature over the past four years, and present the proposed design of a therapeutic HIV vaccine efficacy study by the Department of Veterans Affairs Cooperative Studies Program. We explore the reasons for the atypical design of many vaccine trials, particularly the analysis of efficacy and how it differs from the more usual intent-to-treat analysis used in nonvaccine trials.

Characterization of a recombinant pneumolysin and its use as a protein carrier for pneumococcal type 18C conjugate vaccines

Pneumolysin from Streptococcus pneumoniae was expressed in Escherichia coli as a glutathione S-transferase fusion protein and purified by affinity and hydroxylapatite chromatography. The purified recombinant pneumolysin (rPL), with a molecular mass of 53 kDa, had a specific activity of 3 x 10(5) hemolytic units per mg of protein on rabbit erythrocytes and reacted identically in immunodiffusion with the antisera against native pneumolysin. The rPL was used as a protein carrier to prepare conjugate vaccine with pneumococcal type 18C polysaccharide (PS18C). The PS18C was directly coupled to rPL by reductive animation or was indirectly coupled to rPL via a spacer molecule, adipic acid dihydrazide. The conjugates were nontoxic for mice and guinea pigs at 100 micrograms per dose. The immunogenicity and protective efficacy of both conjugates were tested in mice. A single dose of either of the vaccines elicited a rise in immunoglobulin G antibody production; after two booster injections of the vaccines, statistically significant booster responses (P < 0.001) to both rPL and PS18C were produced. The sera containing the antibodies to rPL were capable of neutralizing the hemolytic activity of rPL to rabbit erythrocytes and the cytotoxicity of rPL to bovine pulmonary endothelial cells. Immunization with the conjugate vaccines conferred statistically significant protection in mice against lethal challenge with type 18C pneumococci.

Avidity of specific IgG antibodies elicited by immunisation against Haemophilus influenzae type b

AIM

To investigate the avidity of specific IgG polyribosyl ribitol phosphate (PRP) antibodies induced by three Haemophilus influenzae type b (Hib) conjugate vaccines: PRP-meningococcal outer membrane protein complex (PRP-OMP), PRP-non-toxic mutant diphtheria toxin, CRM197 (HbOC) and PRP-tetanus toxoid (PRP-T).

METHODS

One hundred and ten infants were immunised with one of the vaccines at two, three and four months of age. Blood samples were taken after each vaccination and serum stored at -20 degrees C. Serum samples collected after the third course (that is, at five months of age) were submitted to antibody avidity assessment, using a urea enzyme linked immunosorbent assay (ELISA).

RESULTS

All three conjugate vaccines elicited IgG PRP antibodies of high median avidity. The resultant antibody populations were heterogeneous with regard to avidity, which in turn was independent of PRP antibody concentration.

CONCLUSIONS

With the recent findings of a correlation between bactericidal killing and affinity, our data highlight the need for a protective level to be expressed qualitatively as well as quantitatively.

Enhanced antibody responses in infants given different sequences of heterogeneous Haemophilus influenzae type b conjugate vaccines

To evaluate the safety and immunogenicity of differing sequences of heterogeneous Haemophilus influenzae type b (Hib) conjugate vaccines, we randomly assigned 300 infants to one of six vaccination schedules. At 2, 4, and 6 months of age, subjects were given single or heterogeneous vaccines: Hib polysaccharide (PRP) conjugated to mutant diphtheria toxin (HbOC), PRP conjugated to outer-membrane protein of Neisseria meningitidis (PRP-OMP), or PRP conjugated to tetanus toxoid (PRP-T). No serious reactions were attributable to immunization with heterogeneous vaccines, and there were few significant differences in the rates of minor adverse reactions among groups. PRP-OMP was the only vaccine that induced an antibody response after the first dose, but significant booster responses were not seen after the second and third doses. Subjects given PRP-T vaccine responded well after two doses, but three doses of HbOC vaccine were needed for an equivalent antibody response. All the Hib vaccine schedules evaluated were immunogenic, and schedules initiated by PRP-OMP vaccine at 2 months of age, followed by two doses of either HbOC or PRP-T vaccine at 4 and 6 months of age, induced the highest antibody levels after each dose. Such schedules may be the best for protecting infants and children who are at greatest risk of having invasive Hib disease, such as American Indian children.

Opposite effects of actively and passively acquired immunity to the carrier on responses of human infants to a Haemophilus influenzae type b conjugate vaccine

Vaccination of infants with Haemophilus influenzae type b (Hib) capsular polysaccharide (HibCP) coupled to carrier proteins has proven protective against invasive Hib diseases in several trials. However, insufficient immunogenicity has been noted in certain populations. Therefore, studies analyzing factors influencing the antibody response to conjugate vaccines are needed. In this study, the response to HibCP coupled to tetanus toxoid (TT) was examined in relation to (i) priming with or coadministration of the carrier protein and (ii) the levels of passively acquired maternal TT antibodies. One hundred forty-four infants were vaccinated with HibCP-TT at 5 and 6 months. They were randomized into three groups that received TT as part of a diphtheria-tetanus-polio vaccine at either 6 and 7 months (group A), 5 and 6 months (group B), or 4 and 5 months (group C). Maternally acquired TT antibodies inhibited the anti-HibCP response to the first HibCP-TT dose in groups A and B (r = -0.5 and -0.4, respectively; P < 0.005). In these groups, infants with prevaccination anti-TT levels above the median failed to reach the defined long-term protective level of HibCP antibodies (1 microgram/ml) more often than infants with low prevaccination levels after the first (P = 0.0001) and the second (P = 0.01) doses of HibCP-TT. In contrast, active priming with TT at 4 months resulted in a threefold-higher median level of anti-HibCP (group C; 1.34 micrograms/ml) than in the unprimed group (group A; 0.40 microgram/ml) after the first dose of HibCP-TT (P = 0.01). Coadministration of TT had no enhancing effect (group B; 0.58 microgram/ml). No significant differences between the median anti-HibCP levels were seen after the second HibCP-TT dose (6.72, 9.63, and 11.44 micrograms/ml in groups A, B, and C, respectively; P = 0.25).