Antibody persistence and Haemophilus influenzae type b carriage after infant immunisation with PRP-T

HIV-Exposed Uninfected Infants Show Robust Memory B-Cell Responses in Spite of a Delayed Accumulation of Memory B Cells: an Observational Study in the First 2 Years of Life

Improved HIV care has led to an increase in the number of HIV-exposed uninfected (HEU) infants born to HIV-infected women. Although they are uninfected, these infants experience increased morbidity and mortality. One explanation may be that their developing immune system is altered by HIV exposure, predisposing them to increased postnatal infections. We explored the impact of HIV exposure on the B-cell compartment by determining the B-cell subset distribution, the frequency of common vaccine antigen-specific memory B cells (MBCs), and the levels of antibodies to the respective antigens in HEU and HIV-unexposed uninfected (HUU) infants born to uninfected mothers, using flow cytometry, a B-cell enzyme-linked immunosorbent spot assay, and an enzyme-linked immunosorbent assay, respectively, during the first 2 years of life. For the majority of the B-cell subsets, there were no differences between HEU and HUU infants. However, HIV exposure was associated with a lower proportion of B cells in general and MBCs in particular, largely due to a lower proportion of unswitched memory B cells. This reduction was maintained even after correcting for age. These phenotypic differences in the MBC compartment did not affect the ability of HEU infants to generate recall responses to previously encountered antigens or reduce the antigen-specific antibody levels at 18 months of life. Although HIV exposure was associated with a transient reduction in the proportion of MBCs, we found that the ability of HEU infants to mount robust MBC and serological responses was unaffected.

Glycoconjugate vaccines: an update

INTRODUCTION

Globally, the three main pathogens causing serious infections are Haemophilus influenzae type b, Streptococcus pneumoniae and Neisseria meningitidis. Over the last 5 years, new vaccines protecting against these bacteria have been developed and introduced in various countries.

AREAS COVERED

This review describes the recently licensed glycoconjugates being used to protect against these encapsulated bacteria. Immunogenicity and safety data that led to licensure or licensure expansion of these glycoconjugates are discussed in addition to the resultant impact on the disease burden.

EXPERT OPINION

The maintenance of robust immunisation programmes with high uptake rates is important in maintaining low rates of disease. Epidemiological surveillance systems are essential in monitoring any changes in infectious disease trends and in identifying emerging infections such as from non-typeable H. influenzae, pneumococcal serotype replacement disease and changes in the epidemiology of meningococcal serogroups. This is important to guide future vaccine development. Accessibility of these glycoconjugate vaccines in resource poor regions, which bear the highest disease burden from these pathogens, remains challenging largely due to high vaccine pricing. Recent aids from public and private funding, tiered vaccine pricing and the transfer of vaccine technology have helped in introducing these vaccines where they are most needed.

Long-term persistence of immunity and B-cell memory following Haemophilus influenzae type B conjugate vaccination in early childhood and response to booster

BACKGROUND

Protection against Haemophilus influenzae type b (Hib), a rapidly invading encapsulated bacteria, is dependent on maintenance of an adequate level of serum antibody through early childhood. In many countries, Hib vaccine booster doses have been implemented after infant immunization to sustain immunity. We investigated the long-term persistence of antibody and immunological memory in primary-school children following infant (with or without booster) Hib vaccination.

METHODS

Anti-polyribosylribitol phosphate (PRP) immunoglobulin G (IgG) concentration and the frequency of circulating Hib-specific memory B cells were measured before a booster of a Hib-serogroup C meningococcal (MenC) conjugate vaccine and again 1 week, 1 month, and 1 year after the booster in 250 healthy children aged 6-12 years in an open-label phase 4 clinical study.

RESULTS

Six to 12 years following infant priming with 3 doses of Hib conjugate vaccine, anti-PRP IgG geometric mean concentrations were 3.11 µg/mL and 0.71 µg/mL and proportions with anti-PRP IgG ≥1.0 µg/mL were 79% and 43% in children who had or had not, respectively, received a fourth Hib conjugate vaccine dose (mean age, 3.9 years). Higher baseline and post-Hib-MenC booster responses (anti-PRP IgG and memory B cells) were found in younger children and in those who had received a fourth Hib dose.

CONCLUSIONS

Sustained Hib conjugate vaccine-induced immunity in children is dependent on time since infant priming and receipt of a booster. Understanding the relationship between humoral and cellular immunity following immunization with conjugate vaccines may direct vaccine design and boosting strategies to sustain individual and population immunity against encapsulated bacteria in early childhood. Clinical Trials Registration ISRCTN728588998.

The continuing role of Haemophilus influenzae type b carriage surveillance as a mechanism for early detection of invasive disease activity

Prior to the introduction of Haemophilus influenzae type b (Hib) conjugate vaccines, Hib was the leading cause of bacterial meningitis in children under five years of age worldwide. In countries that have adopted Hib vaccination schedules, invasive disease has reduced markedly. Oro-naso pharyngeal carriage is recognized as the most significant source of infection. Hib carriage is significantly associated with poverty, such as overcrowding, poor ventilation in houses, lack of running water, and high smoking rates. Additionally, many Indigenous minority groups report high rates of Hib carriage. A resurgence of Hib disease among Alaskan children in the 1990s, lead to a change in approach to eliminate Hib disease and carriage in high-risk populations. This new approach identifies strategies for eliminating Hib disease focusing on the reservoirs of colonization within families and communities. Monitoring Hib carriage continues to offer an early warning system, whereby intervention could prevent invasive disease resurgence.

Haemophilus influenzae type b carriage in Indigenous children and children attending childcare centers in the Northern Territory, Australia, spanning pre- and post-vaccine eras

This paper investigates Haemophilus influenzae type b (Hib) carriage in Indigenous children and children attending childcare centres in the Northern Territory of Australia, 1992-2005. More than 10 years after the introduction of the Hib conjugate vaccine, Hib carriage persists in Indigenous children (3.4%, 2003-2005) and children attending childcare centres (0.2%, 2004). This is the first Australian study to examine Hib carriage spanning the pre- and post-vaccination eras. Increasing carriage rates contribute to Hib disease resurgence especially in those with higher disease burdens, such as remote Indigenous Australians, ongoing carriage surveillance provides a sentinel warning system for invasive disease.

BACKGROUND

Following the introduction of H. influenzae type b (Hib) conjugate vaccine to Australia in 1993 as a three dose (2, 4, 12 month) schedule, the incidence of Hib disease decreased dramatically in children, especially in those aged under 5 years. We investigate Hib carriage in Indigenous children and children attending childcare centres from the Northern Territory (NT) of Australia between 1992 and 2005. We report Hib carriage rates in this well vaccinated population, with previously documented high rates of invasive disease.

METHODS

Hib carriage was reviewed in nasopharyngeal or nasal swabs collected between 1992 and 2005; from over 2000 children (61% Indigenous) aged 0-6 years enrolled in 7 otitis media or carriage surveillance studies in the NT.

RESULTS

More than 10 years after the introduction of the Hib conjugate vaccine, Hib carriage persists at low levels, but at a higher rate in Indigenous children (3.4%, 2003-2005) than children attending childcare centres (0.2%, 2004), in the NT of Australia.

CONCLUSIONS

This is the first Australian study to examine Hib carriage spanning the pre- and post-vaccination eras. Ongoing carriage surveillance provides a sentinel warning system for invasive disease. This notifies public health professionals of potential invasive disease, creating early warning systems for intervention if Australian Indigenous children or children attending childcare centres are to follow current international trends associated with high rates of carriage preceding invasive disease-despite high vaccination rates. Internationally there is growing concern that increasing carriage rates are the driving force behind Hib disease resurgence especially in those with higher disease burdens, such as remote Indigenous Australians. Changes to the vaccination schedule from PRP-OMPC (PedvaxHIB(®)), to PRP-T (2, 4, 6, 12 months) from January 2010-may affect carriage and in time, invasive disease rates. This work is important for national and International comparisons as well as feeding back information to vaccine policy makers of the Hib carriage environment throughout this period.

Immunogenicity and safety of two different Haemophilus influenzae type b conjugate vaccines in Korean infants

The incidence of invasive diseases, including meningitis caused by Haemophilus influenzae type b (Hib) was markedly decreased after routine immunization of Hib vaccine through diverse schedules in many countries. The purpose of this study was to evaluate the immunogenicity and safety of Hib conjugate vaccines in Korean children before the implementation of a national immunization program against Hib in Korea. A multicenter controlled trial was performed on two different Hib vaccines in Korean children. A total of 319 infants were enrolled: 199 infants were immunized with the Hib polysaccharide conjugated to the tetanus toxoid (PRP-T) and 120 infants with the Hib polysaccharide conjugated to the outer-membrane protein of Neisseria meningitides (PRP-OMP). Immunogenicity was evaluated by enzyme-linked immunosorbent assay (ELISA) and serum bactericidal assay. Both vaccines showed good immunologic responses after primary immunization. After 2 doses of PRP-T or PRP-OMP, 78.9% and 91.7% of infants achieved an antibody level of >or=1.0 microg/mL, respectively. Both vaccines were safe and well-tolerated. No serious adverse events were observed. Thus, Hib conjugate vaccines appear to be safe and show good immunogenicity in Korean infants. These results will be important reference data for the implementation of Hib vaccine in the national immunization program of Korea.

Haemophilus influenzae type b vaccine failure in children is associated with inadequate production of high-quality antibody

BACKGROUND

Despite the excellent immunogenicity of Haemophilus influenzae type b (Hib) conjugate vaccines, breakthrough cases of Hib disease still affect a small proportion of vaccinated children in the United Kingdom. We performed a retrospective study to compare the avidity of antibody directed against the Hib polysaccharide capsule (PRP) in children who experienced Hib vaccine failure in the United Kingdom among 3 historical cohorts and with age-matched healthy control subjects.

METHODS

Serum samples from vaccinated children with invasive Hib disease were collected beginning in 1992 as part of enhanced surveillance for Hib disease following vaccine introduction. A total of 251 children who experienced Hib vaccine failure were identified from 3 historical cohorts (1992-1995, 1996-1999, and 2000-2003). The anti-PRP antibody concentration and avidity from healthy age-matched control subjects was obtained for the 3 contemporary time points (1995, 1999, and 2002). Serum anti-PRP antibody concentration was measured in each of the samples using a standard Hib ELISA, and antibody avidity was determined using thiocyanate elution.

RESULTS

Within the first 60 days after disease onset, there was no change in the anti-PRP antibody avidity, and there was no statistically significant difference in the geometric mean Hib antibody avidity over the 3 study periods. However, the children who experienced Hib vaccine failure had significantly lower Hib antibody avidity than did healthy control subjects, despite a marked antibody response following infection.

CONCLUSIONS

Children who experience Hib disease despite vaccination appear to have a defect in immunological priming, leading to a qualitative difference in Hib-specific memory B cells. Low anti-PRP antibody avidity decreases the functional activity of anti-PRP antibody in the sera of these children experiencing vaccine failure, leading to disease susceptibility.

Bacterial meningitis: the impact of vaccination

Acute bacterial meningitis remains an important cause of morbidity and mortality in children. Children <2 years of age are particularly susceptible to infection with encapsulated bacteria due to their immature response to polysaccharide antigens. Conjugate vaccines, which induce T cell memory, can provide immunological protection for these children. The Haemophilus influenzae type b (Hib) conjugate vaccine was the first such vaccine to become available. The efficacy of the vaccine has been quoted as being 98%. Its introduction was followed by a dramatic decrease in the incidence of all invasive Hib disease, including meningitis. This reduction was in part due to the ability of these vaccines to reduce nasopharyngeal carriage of the organism and thereby induce herd immunity. Different Hib vaccines use a variety of protein carriers and differ in their immunogenicity and efficacy. The most suitable vaccine needs to be determined according to the local epidemiology of Hib disease. Commercial combination vaccines may lead to lower antibody levels. A recent increase in the incidence of Hib disease in the UK highlights the importance of continued surveillance and the need for booster vaccinations to ensure continued protection. Conjugate vaccines to Streptococcus pneumoniae and Neisseria meningitidis have been developed. The introduction of a pneumococcal conjugate vaccine in the US has led to a decrease in the rate of infection by nearly 60% in children <5 years of age. A reduction in pneumococcal carriage may also modify disease epidemiology. The UK introduced the conjugate meningococcal C vaccine into its infant schedule with a corresponding reduction in N. meningitidis group C disease. A recent decrease in the effectiveness of the vaccine, however, suggests a booster may be necessary in the future. Our present understanding of the immunology of conjugate vaccines is far from complete. Developed countries have introduced conjugate vaccines into their immunisation schedules to prevent bacterial meningitis; however, their high cost precludes their use in many developing countries. Progress needs to be made in order to get these highly effective vaccines to those areas that need them.

Haemophilus influenzae type b reemergence after combination immunization

Combination vaccines may suppress Hib antibody concentration and avidity.

An increase in Haemophilus influenzae type b (Hib) in British children has been linked to the widespread use of a diphtheria/tetanus/acellular pertussis combination vaccine (DTaP-Hib). We measured anti-polyribosyl-ribitol phosphate antibody concentration and avidity before and after a Hib booster in 176 children 2–4 years of age who had received 3 doses of DTP-Hib (either DT whole cell pertussis-Hib or DTaP-Hib) combination vaccine in infancy. We also measured pharyngeal carriage of Hib. Antibody concentrations before and avidity indices after vaccination were low (geometric mean concentration 0.46 μg/mL, 95% confidence interval [CI] 0.36–0.58; geometric mean avidity index 0.16, 95% CI 0.14–0.18) and inversely related to the number of previous doses of DTaP-Hib (p = 0.02 and p<0.001, respectively). Hib was found in 2.1% (95% CI 0.7%–6.0%) of study participants. Our data support an association between DTaP-Hib vaccine combinations and clinical Hib disease through an effect on antibody concentration and avidity.

Haemophilus influenzae type b immunization in infants in the United Kingdom: effects of diphtheria/tetanus/acellular pertussis/Hib combination vaccine, significant prematurity, and a fourth dose

OBJECTIVE

To measure anti-polyribosylribitolphosphate (PRP) antibody and anti-tetanus toxoid (TT) antibody responses in UK infants to explore the effects of (1) immunization with an acellular diphtheria/tetanus/pertussis/Haemophilus influenzae type b (DTPHib) combination vaccine, (2) significant preterm delivery, and (3) a fourth dose of conjugated Hib vaccine (PRP-T) in those with a low anti-PRP antibody (<1.0 microg/mL) after primary immunization.

METHODS

A prospective study was conducted in 4 tertiary neonatal units at a time when 2 types of DTPHib vaccines were used interchangeably in the United Kingdom for primary immunization: acellular (DTPaHib) and whole cell. Timing and type of all vaccine doses were as per standard UK practice. Blood was taken before and after immunization. A total of 166 preterm and 45 term infants completed the study; 97 (15 term) infants who had anti-PRP antibody <1.0 microg/mL were offered a fourth dose of PRP-T; 61 (55 preterm) then had repeat antibody measurements. Anti-PRP and anti-TT antibody after primary immunization relative to gestation and number of whole cell vaccine doses received was measured, as well as anti-PRP antibody after a fourth dose of PRP-T.

RESULTS

A total of 49% of preterm and 33% of term infants had anti-PRP antibody <1.0 microg/mL after full primary immunization. Receipt of 1 or more acellular vaccine doses was associated with lower anti-PRP antibody, a dose response effect being observed. Preterm infants were less likely to have anti-PRP antibody >1.0 microg/mL compared with term infants. A total of 93% of infants who were given a fourth dose had anti-PRP antibody >1.0 microg/mL. Anti-TT antibody responses were satisfactory for all infants but also reduced by each DTPaHib dose received.

CONCLUSION

Infants who receive DTPaHib, are significantly preterm, or who do not receive a fourth dose of conjugated Hib vaccine may be at increased risk for Hib disease.

Haemophilus influenzae type b conjugate vaccines

Haemophilus influenzae type b (Hib) is one of the leading causes of invasive bacterial infection in young children worldwide. During childhood, acquisition of antibody directed against the polysaccharide capsule of the organism, presumably as a result of asymptomatic carriage, confers protection and disease is much less common after the age of 4 years. Like other polysaccharides, the polyribosyl ribitol phosphate (PRP) of the Hib capsule is a T-independent antigen and not immunogenic when administered as a vaccine in infancy. Because the highest rates of disease occur in the first 2 years of life, efficacious Hib vaccines have been designed by covalently linking the PRP capsule to a carrier protein that recruits T-cell help for the polysaccharide immune response and induces anti-PRP antibody production even in the first 6 months of life. Introduction of Hib protein-polysaccharide conjugate vaccines into many industrialized countries over the past 15 years has resulted in the virtual elimination of invasive Hib disease. However, despite the success of the vaccine programme several factors may interfere with the effectiveness of the vaccine in the routine programme, as observed in the UK recently. Such factors may include interference with other concomitant vaccines, waning immunity in the absence of booster doses of vaccine, and reduced natural boosting as a result of decreased transmission of the organism. However, the burden of disease remains highest in resource-poor countries and urgent efforts are needed to provide the benefits of this vaccine for children living in regions where it cannot be used for economic and logistical reasons.

Vaccines against bacterial meningitis

Meningitis remains an important cause of morbidity and mortality among children >5 years of age and is especially prevalent in developing countries. Effective routine immunization against Hib, pneumococcus and serogroupC meningococcus has had a significant impact on both invasive disease and carriage caused by these encapsulated bacteria. The major challenge in prevention of meningitis remains the delivery of vaccines worldwide, especially to resource-poor regions with the greatest disease burden.

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

On September 22 to 25, 2002, a group of infectious disease specialists, public health officials, and vaccine experts from 33 countries gathered in Scottsdale, Arizona, to discuss the epidemiology and control of disease caused by Haemophilus influenzae type b (Hib) in the era of Hib conjugate vaccines. This supplement is a synthesis of the major themes and key lessons identified at the meeting. The objectives of the conference were to review the 10-year experience with Hib conjugate vaccines, discuss strategies to reduce Hib disease rates to lowest possible levels in industrialized countries, review impediments to the introduction of Hib vaccine in developing countries, and discuss strategies for disseminating lessons learned from countries using to those not using Hib conjugate vaccines. Over 10 years of international experience with Hib conjugate vaccines has demonstrated that they are safe and effective. Routine use of Hib conjugate vaccine has consistently led to decreases in the incidence of invasive Hib disease of 90% or more across a wide range of epidemiologic situations in industrialized countries. In some countries, the vaccine has caused a near-disappearance of invasive Hib disease through a combination of direct protection and herd immunity. Developing countries that have implemented routine vaccination (eg, The Gambia, Chile) have also had substantial disease reduction. In countries where Hib conjugate vaccine is being used, reducing Hib disease incidence to the lowest possible level will depend on maintaining high vaccine coverage levels, conducting surveillance for Hib disease, and investigating Hib disease cases. The optimal Hib vaccination strategy will depend on many factors, including local epidemiology and programmatic considerations. In countries that are not using Hib conjugate vaccine, information on the local burden of Hib disease will be essential for leaders considering vaccine introduction. Where disease burden is high, a multifaceted approach is urgently needed to evaluate and overcome barriers to vaccine introduction. In areas where Hib disease burden is not well characterized, additional work will be needed to understand the epidemiology of Hib disease and to communicate the value of Hib conjugate vaccine.

Long-term persistence of immunity after immunisation with Haemophilus influenzae type b conjugate vaccine

Although Haemophilus influenzae type b (Hib) conjugate vaccines, after licensure in 1987, are now recommended for world-wide use, the duration of protective immunity afforded by them is not known. We therefore assessed the immunogenity at 9-10 years of age in 37 children who had received the first Hib conjugate, PRP-D, in infancy (the Hib-conjugate group) and were now given a dose of Hib polysaccharide (PS) as a test vaccine. The anti-Hib PS antibodies (Hib-ab) were measured before and after this test vaccination, and the values compared to those in 37 control children who had not previously received any Hib vaccine and in 13 children who had received Hib PS vaccine in infancy (the Hib-PS group). Prior to the test vaccination, the Hib-ab concentrations in the Hib-conjugate group were 3.6-fold higher than in the control group. After the test vaccination, the Hib-conjugate group had higher total Hib-ab concentrations, higher proportion of IgG and higher avidity of Hib-ab than the control or the Hib-PS group, suggesting persisting immunological memory in a Hib-c group. A mathematical model, including memory, predicted accurately the Hib-ab concentrations, which are maintained through anamnestic responses to intervening stimuli (Hib or cross-reacting bacteria).

Immunologic memory in Haemophilus influenzae type b conjugate vaccine failure

AIMS

To compare the convalescent antibody response to invasive Haemophilus influenzae type b (Hib) disease between conjugate vaccine immunised and unimmunised children, to look for evidence of priming for immunologic memory.

METHODS

Unmatched case-control study in the UK and Eire 1992-2001 and Victoria, Australia 1988-1990. A total of 93 children were identified as having invasive Hib disease following three doses of conjugate vaccine in infancy through post licensure surveillance throughout the UK and Eire; 92 unvaccinated children admitted to an Australian paediatric hospital with invasive Hib disease were used as historical controls. Convalescent serum was taken for measurement of Hib antibody concentration, and clinical information relating to potential disease risk factors was collected. The geometric mean concentrations of convalescent Hib antibodies were compared between immunised and unimmunised children, using raw and adjusted data.

RESULTS

Hib conjugate vaccine immunised children had higher serum Hib antibody responses to disease (geometric mean concentration (GMC) 10.81 microg/ml (95% CI 6.62 to 17.66) than unimmunised children (1.06 microg/ml (0.61 to 1.84)) (p < 0.0001). However, following adjustment for the significant confounding influences of age at presentation and timing of serum collection, a difference persisted only in children presenting with meningitis (vaccinated GMC 3.78 microg/ml (2.78 to 5.15); unvaccinated GMC 1.48 microg/ml (0.90 to 2.21); p = 0.003).

CONCLUSIONS

Higher antibody responses to invasive Hib disease in vaccinated children with meningitis reflect priming for immunologic memory by the vaccine. Although a majority of children in the UK are protected from Hib disease by immunisation, the relative roles of immunologic memory and other immune mechanisms in conferring protection remain unclear.

Pneumococcal vaccination and revaccination of older adults

As individuals advance in age, the risk of infection, bacteremia, and mortality caused by Streptococcus pneumoniae rises. Retrospective data demonstrate that the licensed penumococcal polysaccharide vaccine (PPV) is effective in older persons in reducing serotype-specific invasive disease. PPV demonstrates good immunogenicity in older adults, generally comparable to that in younger subjects, although certain cohorts respond less well. The response to PPV is T cell independent, however, and does not elicit immunologic memory. The duration of the anti-capsular polysaccharide antibody response appears to wane as early as 3 years after vaccination. In older persons, revaccination induces an antibody response, although it may not be as strong as that from the initial vaccine. While revaccination of older adults has been recommended, clinical efficacy has not yet been proven. Measures of antibody function may be at least as important in determining protection as are quantitative antibody levels. Additional studies of immunogenicity, particularly regarding revaccination, will facilitate the design of an optimal pneumococcal vaccination policy. Research into conjugate- and protein-based pneumococcal vaccines, which elicit T-cell-dependent responses and induce immunologic memory, is needed in older persons. In the meantime, administering to PPV to recommended groups should be a public health priority.

Hib vaccination in infants born prematurely

AIMS

To document the immunogenicity and persistence of antibody to polyribosyl-ribitol phosphate (PRP) as well as the clinical protection against invasive Haemophilus influenzae type b (Hib) disease in premature infants immunised at the routine schedule.

METHODS

Blood was obtained at 2, 5, 12, and 64 months of age from a cohort of prematurely born infants (<or=32 weeks gestation). Anti-PRP antibody concentrations were compared with those of a control cohort of infants born at full term and vaccinated at the same schedule. Hib vaccine failures occurring between October 1992 and October 2000 were reported by paediatricians through an active, prospective, national survey in the UK and Republic of Ireland. The number of prematurely born children with vaccine failure was compared with the corresponding number born at term.

RESULTS

Twenty seven prematurely born infants were followed to 5 years of age. Compared with term infants they had a significantly lower geometric mean concentration of anti-PRP antibody and/or a significantly lower proportion above one or both of the conventional protective antibody concentrations (0.15 and 1.0 micro g/ml) at all ages. A total of 165 cases of invasive Hib disease were identified over eight years of national surveillance. Eighteen were premature (<37 weeks); approximately 12 would be expected. The relative risk of UK premature infants developing disease compared with term infants was 1.5 (95% CI 0.9 to 2.6).

CONCLUSIONS

Premature infants develop lower antibody concentrations than term infants following Hib conjugate vaccination. Premature infants may also have an increased risk of clinical vaccine failure, but interpretation is limited by the small number of premature infants developing invasive Hib disease over eight years of national surveillance. Overall, vaccination with Hib conjugate vaccines affords a high level of protection to premature babies.

The UK Hib vaccine experience

Haemophilus influenzae type b (Hib) is an important pathogen in children <5 years of age. The Hib conjugate vaccines were the first polysaccharide-protein conjugate vaccines to be used in routine childhood schedules. Their introduction in the UK in 1992 has resulted in the near elimination of Hib disease. The UK Hib vaccine programme has a number of unique features including an accelerated schedule, absence of a booster dose, and the inclusion of a catch up component at the beginning of the programme. Collaboration between UK paediatricians, microbiologists, and public health physicians has allowed active national surveillance of Haemophilus influenzae disease and enabled important conclusions to be drawn. These include high vaccine effectiveness, the presence of herd immunity, and the frequency of risk factors in cases of vaccine failure and in cases of invasive disease due to non-b H influenzae. Parallel studies have shown the immunogenicity, induction of immunological memory, and persistence of immunity following vaccination at the UK schedule, as well as measuring the impact of vaccination on pharyngeal Hib carriage. Cases continue to occur and complacency is ill advised, particularly as other vaccines and combinations are introduced. Surveillance of H influenzae disease in the UK remains important.

Cerebrospinal fluid

A quick and accurate diagnosis of maladies affecting the central nervous system (CNS) is imperative. Procurement and analysis of cerebrospinal fluid (CSF) are paramount in helping the clinician determine a patient's clinical condition. Various staining methods, measurement of white blood cell counts, glucose and protein levels, recognition of xanthochromia, and microbiologic studies are CSF parameters that are collectively important in the ultimate determination by a clinician of the presence or absence of a catastrophic CNS condition. Many of these CNS parameters have significant limitations that should be recognized to minimize under treating patients with catastrophic illness.

Immunological Characterization of ConjugatedHaemophilus influenzaeType b Vaccine Failure in Infants

Infant vaccination with conjugated Haemophilus influenzae type b (Hib) vaccine is highly effective in protecting against invasive Hib infections, but vaccine failures do occur. Twenty-one vaccine failures are reported since the introduction of the Hib conjugate vaccine in The Netherlands. Of the 14 evaluable patients, 6 children showed no antibody response to Hib polysaccharide in convalescent-phase serum (immunoglobulin [Ig] G anti-Hib level <1.0 microg/mL), including 1 child with hypogammaglobulinemia and 1 child with IgG2 deficiency. After revaccination, almost all children developed anti-Hib antibodies. In case of Hib vaccine failure, case investigation should be performed, including measurement of serum Ig concentrations as well as specific anti-Hib antibodies. Invasive Hib disease after infant conjugate Hib vaccination may be the presentation of an underlying immunodeficiency, but more often, only a decreased antibody response to Hib is found; revaccination with conjugated Hib vaccine is advised.

Antibodies to Haemophilus influenzae serotype b in the Netherlands a few years after the introduction of routine vaccination

We assessed antibodies to the capsular polysaccharide of Haemophilus influenzae type b (HibPS) in the Dutch population a few years after a mass vaccination against H. influenzae (Hib) was begun. We observed sharp declines in the geometric mean titer (GMT) and the prevalence of HibPS antibodies at levels of < or =0.15 microg/mL in children who had received 4 doses of vaccine: from 8.65 microg/mL (prevalence, 99.4%) after 0-2 months to 0.8 microg/mL (prevalence, 83.3%) after 27-29 months. In adult groups, both the prevalence of HibPS antibodies and the GMT declined significantly with increasing age but remained high (prevalence, > or =83.7%; GMT, 0.73 > or = microg/mL). We conclude that the overall immunity in the Dutch population seems satisfactory. We draw our conclusions from the current serosurveillance study and from the sharp decline in invasive Hib disease noted after the introduction of vaccination. The key questions for the future are (1) whether Hib and cross-reacting organisms will circulate sufficiently to provide natural reexposure, and (2) how long memory immunity will persist after vaccination without reexposure.

Immunogenicity and reactogenicity of a pneumococcal conjugate vaccine administered combined with a haemophilus influenzae type B conjugate vaccine in United Kingdom infants

BACKGROUND

Streptococcus pneumoniae is a major disease burden in young children and the incidence of antibiotic-resistant pneumococcal strains is increasing. Multivalent pneumococcal saccharide-protein conjugate vaccines have recently been developed.

OBJECTIVES

To assess the immunogenicity and reactogenicity of a 7-valent pneumococcal conjugate vaccine (7VPnC) administered as a separate injection or as a combined injection with Haemophilus influenzae type b vaccine (HbOC) at 2, 3 and 4 months of age.

METHODS

Randomized controlled trial of 368 healthy UK infants receiving routine vaccines only (control group), routine vaccines and 7VPnC as a separate injection (separate group), or routine vaccines and 7VPnC combined with HbOC (combined group) at 2, 3 and 4 months. The control group received 7VPnC at 5, 6 and 7 months. All groups received pneumococcal polysaccharide vaccine at 13 to 16 months. Anticapsular IgG antibodies to 7VPnC serotypes were measured at 2, 5, 13 and 14 months and safety data collected.

RESULTS

IgG antibody concentrations at 5 months were higher in the two treatment groups compared with the controls for all 7VPnC serotypes (P < 0.001) and higher in the separate group than the combined group for five 7VPnC serotypes (P < 0.05). For both treatment groups antibody concentrations were higher at 14 months (range, 6.6 to 25.3 microg/ml) than at 5 months (range, 0.6 to 2.5 microg/ml) for all 7VPnC serotypes (P < 0.001).

CONCLUSION

7VPnC was well-tolerated, safe and immunogenic when administered as a separate or as a combined 7VPnC/HbOC injection. Although antibody responses were lower in the infants who received the combination compared with those who received 7VPnC as a separate injection, marked anamnestic responses to polysaccharide challenge were observed, suggesting that both groups were immunologically primed.

Epidemiology of bacterial meningitis

Bacterial meningitis caused by Haemophilus influenzae type B (Hib) has almost disappeared from the United States, Canada, and other countries that have implemented routine vaccination with Hib conjugate vaccines. The overall incidence of meningitis in these countries has declined by more than 50%, and the age distribution of susceptibility has shifted, so that the disease is now more common in adults than in children. Another new feature of the epidemiology of bacterial meningitis has been the occurrence of clusters of meningococcal disease. Such clusters have been school related, mainly in adolescents, and most clusters have been associated with a clone of group C, serotype 2a. The role of cigarette smoking as a risk factor for bacterial meningitis has been confirmed and adds urgency to the efforts to control smoking in adolescents and young adults.

Extended follow-up of antibody levels and antigen responsiveness after 2 Haemophilus influenzae type b conjugate vaccines

Although immunization programs with Haemophilus influenzae type b (Hib) conjugate vaccines have dramatically reduced disease incidence, few data are available regarding the duration of protection after vaccination. We measured serum anti-polyribosylribitol phosphate (PRP) levels in healthy 4- to 5- year-old children previously given 4 doses of PRP-T vaccine (at 2, 4, 6, and 18 months) or 1 dose of PRP-D vaccine (at 19 months) during clinical trials to assess antibody persistence. Concurrent with other preschool immunizations, half of the children were randomly assigned to receive a PRP-T booster immunization to assess responsiveness. Among 136 subjects who were primed with PRP-D, the baseline geometric mean concentration of antibody was 0.7 microg/mL (95% CI 0.5 to 0.9). Concentrations were <0.15 microg/mL in 24 (17.6%) subjects. Among 212 children who were primed with PRP-T, the geometric mean concentration was 2.2 microg/mL (95% CI 1.9 to 2.5) (P <.001). Only 2 (0.9%) had concentrations <0.15 microg/mL. Four weeks after PRP-T immunization, geometric mean concentrations had increased to 98.4 and 102.0 microg/mL, respectively. Responses were strong even in those with low or undetectable preimmunization antibody levels. Spontaneous increases in antibody levels were seen in 9 (5.2%) of 172 subjects not given additional PRP-T. We concluded that among 4- to 5-year-olds, anti-PRP levels remained above 0.15 microg/mL in nearly all children after PRP-T priming and in most after PRP-D priming, and that both groups were able to respond vigorously to restimulation, consistent with persistent immune memory.