Global reduction of Hib disease: what are the next steps? Proceedings of the meeting Scottsdale, Arizona, September 22-25, 2002

Complex role of hemoglobin and hemoglobin-haptoglobin binding proteins in Haemophilus influenzae virulence in the infant rat model of invasive infection

Haemophilus influenzae requires an exogenous heme source for aerobic growth in vitro. Hemoglobin or hemoglobin-haptoglobin satisfies this requirement. Heme acquisition from hemoglobin-haptoglobin is mediated by proteins encoded by hgp genes. Both Hgps and additional proteins, including those encoded by the hxu operon, provide independent pathways for hemoglobin utilization. Recently we showed that deletion of the set of three hgp genes from a nontypeable strain (86-028NP) of H. influenzae attenuated virulence in the chinchilla otitis media model of noninvasive disease. The present study was undertaken to investigate the role of the hgp genes in virulence of the wild-type serotype b clinical isolate HI689 in the infant rat model of hematogenous meningitis, an established model of invasive disease requiring aerobic growth. Bacteremia of high titer and long duration (>14 days) and histopathologically confirmed meningitis occurred in >95% of infant rats challenged at 5 days of age with strain HI689. While mutations disrupting either the Hgp- or Hxu-mediated pathway of heme acquisition had no effect on virulence in infant rats, an isogenic mutant deficient for both pathways was unable to sustain bacteremia or produce meningitis. In contrast, mutations disrupting either pathway decreased the limited ability of H. influenzae to initiate and sustain bacteremia in weanling rats. Biochemical and growth studies also indicated that infant rat plasma contains multiple heme sources that change with age. Taken together, these data indicate that both the hgp genes and the hxuC gene are virulence determinants in the rat model of human invasive disease.

Reactogenicity and immunogenicity profiles of a novel pentavalent diphtheria-tetanus-whole cell pertussis-hepatitis B and Haemophilus influenzae type B vaccine: a randomized dose-ranging trial of the Hib tetanus-conjugate content

BACKGROUND

Combined vaccines containing diphtheria-tetanus-pertussis whole-cell (DTPw), Haemophilus influenzae type b (Hib), and hepatitis-B vaccines are essential for the continuing success of vaccination programs in developing nations. This randomized, dose-ranging study assessed the immunogenicity and reactogenicity of primary and booster vaccination with pentavalent DTPw-HBV/Hib vaccines containing 10, 5 or 2.5 microg of polyribosylribitol phosphate (PRP) conjugated to tetanus toxoid (trials Hib-052/064).

METHODS

Six hundred eighty infants were randomized to receive one of 5 vaccine combinations at 6, 10, and 14 weeks of age. Of these, 351 received the same vaccine at 15-24 months of age. The immune response was evaluated on blood samples collected 1 month after the 3-dose primary course and before and 6 weeks after the booster dose. Reactogenicity was assessed during a 4-day period after each vaccine dose using diary cards.

RESULTS

After primary vaccination, all subjects had seroprotective anti-PRP antibody concentrations (> or = 0.15 microg/mL) and > 95% had concentrations > or = 1.0 microg/mL, irrespective of the PRP dose administered. Anti-PRP antibody avidity after primary vaccination and antibody persistence until the second year of life were similar among groups. The booster dose induced marked increases in anti-PRP antibody GMCs and antibody avidity, indicative of effective priming and the presence of immune memory. All vaccination regimens elicited good immune responses and comparable antibody persistence to the other vaccine antigens, with significant increases in all antibody concentrations observed after boosting. All vaccination regimens were safe, with similar overall reactogenicity profiles.

CONCLUSION

Hib conjugate vaccines containing reduced amounts of PRP can be effectively combined with the licensed DTPw-HBV vaccine to provide protection against 5 major childhood pathogens in a single injection.

Efficacy of Haemophilus influenzae type b vaccination of children: a meta-analysis

Haemophilus influenzae type b (Hib) infection is a leading cause of meningitis and pneumonia in infants and children in developing countries, and yet the implementation of routine Hib vaccination is very slow. The aim of the present study was to quantify the protective efficacy of H. influenzae type b vaccination of young children. MEDLINE, EMBASE, and the Cochrane Central Register of Controlled Trials Register were searched. References of selected articles were also reviewed and experts contacted. Eight randomized trials were found that compared the efficacy of H. influenzae type b conjugate vaccine to placebo or no vaccine. Information on study design, patients enrolled, age, vaccine type, cases of invasive H. influenzae type b disease, adverse events, and items to assess potential for bias was recorded. The incidence of invasive H. influenzae type b infection formed the primary outcome. The odds ratio (OR) of developing Hib infection was combined using a random effects model to provide a measure of vaccine efficacy. The protective effect, defined as the relative risk reduction, was estimated as (1-OR). From eight trials, the protective efficacy of the Hib conjugate vaccine was 84% (OR 0.16; 95%CI 0.08-0.30) against invasive Hib disease, 75% (OR 0.25; 95%CI 0.08-0.84) against meningitis, and 69% (OR 0.31; 95%CI 0.10-0.97) against pneumonia. Serious adverse events were rare. The results provide firm evidence that Hib conjugate vaccines are safe and effective in reducing the risk of all forms of invasive Hib disease, further establishing that vaccination of children in developing countries can protect them from a potentially fatal yet preventable disease.

Influence of genetic and environmental factors on the immunogenicity of Hib vaccine in Gambian twins

The differences in incidence rates of Haemophilus influenzae type b disease and the variation in Hib conjugate vaccine efficacy achieved among different ethnic groups suggest genetic influences on the immune response to Hib vaccine. The serum anti-PRP antibody concentration of 43 monozygotic (MZ) and 147 dizygotic (DZ) twin pairs in the Gambia was measured using a standardised Hib ELISA. Intrapair correlations for MZ and DZ twin pairs were compared and heritability in antibody responses to Hib conjugate vaccine was estimated to be 51% (95% CI: 32-66%), indicating a significant genetic contribution in the response. We conclude that genetic factors may be involved in the variation in immune response to Hib vaccine observed in different populations and may contribute to cases of vaccine failure.

Development Of Priority Vaccines For Disease-Endemic Countries: Risk And Benefit

Decisions regarding vaccine regulation and use made by institutions in industrialized countries can have an unintended impact on vaccines' availability in disease-endemic countries. However, regulatory and programmatic decision making by such countries, taking into consideration local risks and benefits, requires adequate resources, both human and financial. Such differing risk-benefit determinations between industrialized and disease-endemic countries will increase product divergence. We propose a single universal standard for risk assessment, based on maximizing net benefit, and an action plan to improve access to priority vaccines through a more robust determination of risk and benefit.

Office pediatrics: current issues in lower respiratory infections in children

PURPOSE OF REVIEW

Recent estimates indicate that 1.9 million children worldwide die each year from acute respiratory illnesses, many of which are lower respiratory infections (LRIs). Global threats from indoor and outdoor air pollution, urban crowding, biologic weapons, and worldwide rapid transportation complicate our efforts to reduce the impact of LRI disease in children. As new strategies are developed to limit the spread of LRI disease in children, existing pathogens become more complex to treat and newer pathogens emerge as causes of clinical disease. Most LRI disease is managed in outpatient settings and remains a challenge to those who care for children.

RECENT FINDINGS

Atypical pathogens (notably mycoplasma pneumoniae) are now known to cause a wide spectrum of disease, and with more extensive complications than previously recognized. Available data suggest a rapid increase in S. pneumoniae resistance to penicillin and macrolides in many cases of community acquired pneumonia, which has resulted in shifts in outpatient (and inpatient) antibiotic treatment regimens. Human metapneumovirus (HMPV) is now recognized to cause LRI illness similar to respiratory syncytial virus (RSV) in infants and children. General strategies for the prevention of influenza infection are expanded to include many infants and young children, and are projected to significantly reduce morbidity and mortality from this key LRI pathogen.

SUMMARY

The spectrum of LRI pathogens--and the clinical illnesses caused by them--continues to change. Innovations in immunotherapy and vaccines will be instrumental to reducing the burden of LRI morbidity in infants and children worldwide in the future.

Invasive Haemophilus influenzae type B diseases in Bangladesh, with increased resistance to antibiotics

OBJECTIVE

To determine the prevalence, age-group distribution, serotype, and antibiotic susceptibility patterns of invasive Haemophilus influenzae type b (Hib) isolates in Bangladeshi children because data regarding Hib diseases in developing countries are scarce, which has led to delay of the introduction of Hib vaccine in these countries.

METHODS

Children diagnosed with meningitis (n = 1412) and pneumonia (n = 2434) were enrolled in this surveillance study for Hib invasive diseases. Cerebrospinal fluid (CSF) and blood specimens, and the subsequent isolates, were processed using standard procedures.

RESULTS

During 1993 to 2003, 455 H influenzae strains were isolated from patients with meningitis (n = 425) and pneumonia (n = 30), and an additional 68 Hib meningitis cases were detected by latex agglutination (LA) testing. Overall, 35% of pyogenic meningitis cases were a result of H influenzae, 97.1% of which were Hib. Most (91.4%) cases occurred during the first year of life. Resistance to ampicillin, chloramphenicol, and cotrimoxazole was 32.5%, 21.5%, and 49.2%, respectively. There was a trend toward increasing resistance for all three drugs. Resistance to ampicillin and chloramphenicol was almost universally coexistent and was associated with increased sequelae compared with the patients infected with susceptible strains (31% [23/75] vs 11% [21/183]; P <.001).

CONCLUSION

Hib is the most predominant cause of meningitis in young Bangladeshi children. Resistance to ampicillin and chloramphenicol and the high cost of third-generation cephalosporin highlight the importance of disease prevention through vaccination against Hib.

Dose response of CRM197 and tetanus toxoid-conjugated Haemophilus influenzae type b vaccines

High vaccine cost has limited use of conjugate vaccines in the developing world where the disease burden is greatest. Fixed fractional doses of Haemophilus influenzae type b (Hib) vaccines have been shown to be immunogenic, but dose responses of these vaccines in humans are needed to determine the lowest immunogenic dose as an option for lowering vaccine cost. We randomized children to receive one of five doses (0.625, 1.25, 2.5, 5.0 and 10 microg) of either a diphtheria CRM197 or tetanus toxoid-conjugated Hib vaccine. The children received a primary three-dose series at 6, 10, and 14 weeks of age and a booster dose at 9 months. Anti-PRP IgG antibodies were measured at each vaccination, at 18 weeks, and at one week following the booster dose. Concentrations of > or =1.25 microg of HibCRM197 vaccine produced mean anti-PRP responses at 18 weeks of > or =5.72 microg/ml and > or =0.15 microg/ml was achieved in >98% of the children with at least 79% reaching anti-PRP concentrations of > or =1.0 microg/ml. Concentrations of > or =1.25 microg of Hib-tetanus vaccine produced mean anti-PRP responses at 18 weeks of > or =8.63 microg/ml and > or =0.15 microg/ml was achieved in 100% of the children with at least 88.9% reaching anti-PRP concentrations of > or =1.0 microg/ml. While mean antibody concentrations after either vaccine decreased over time, the proportion of children with antibody levels of > or =0.15 microg/ml had not changed significantly at the 9 month measurement. Immunologic memory was demonstrated by significant increases in mean antibody concentrations one week after the booster dose for doses > or =1.25 microg of HibCRM197 and Hib-tetanus to mean concentrations > or =37.71 and 16.07 microg/ml, respectively. There were no differences in antibody responses for vaccine doses > or =1.25 microg of the same vaccine or between the same concentrations of the two different vaccines. Our data suggest that doses of these vaccines of > or =1.25 microg may be sufficient to stimulate an immune response that offers both short and longer term protection from invasive Hib disease.