Meningococcal glycoconjugate vaccines

Abstract

Neisseria meningitidis is a major cause of invasive bacterial infections worldwide. For this reason, efforts to control the disease have been directed at optimizing meningococcal vaccines and implementing appropriate vaccination policies. In the past, plain polysaccharide vaccines containing purified capsular polysaccharides A, C, Y and W135 were developed, but failed to protect infants, who are at greatest risk. Experience with the conjugate Haemophilus vaccine suggested that this approach might well empower meningococcal vaccines. Thus, a very efficacious vaccine against serogroup C Neisseria meningitis was optimized and has been widely used in developed nations since 1999. On the basis of epidemiological changes in the circulation of pathogenic serogroups in the United States, a quadrivalent conjugate vaccine against A, C, Y and W135 serogroups (Menactra™) has been developed and was approved by the U.S. FDA (Food and Drug Administration) in 2005. Recently, another tetravalent conjugate meningococcal vaccine (Menveo™) has been licensed and made available in the United States of America and in the European Union. Finally, in response to large epidemics caused by serogroup A meningococcus in Africa, a new, safe, immunogenic and affordable vaccine has been developed. This review highlights the evolution of conjugate meningococcal vaccines in general and discusses how this kind of vaccine can contribute to preventing meningococcal disease.

Introduction

Although two centuries have passed since Vieusseux first described epidemic meningococcal disease,1 Neisseria meningitidis remains a major and insidious cause of death, even in industrialized countries. Indeed, meningococcal disease can develop extremely rapidly and is associated with a high case-fatality rate, although antibiotics, such as rifampicin or cephalosporins, usually have great bactericidal efficacy. However, some antibiotic resistance has recently been reported (e.g., ciprofloxacin resistance).2

For this reason, efforts to control the disease have been directed at optimizing meningococcal vaccines and implementing appropriate vaccination policies. Plain polysaccharide vaccines have failed to protect infants, who are at greatest risk. Experience with the conjugate Haemophilus vaccine suggested that this approach might empower meningococcal vaccines too. Thus, a very efficacious vaccine against type C Neisseria meningitidis was optimized and has been widely used in developed nations since 1999.3,4 Multivalent-conjugate (7- 10- and 13-valent) vaccines against S. pneumoniae have also been developed.5–7 Recently, two tetravalent (against A, C, Y and W135 serogroups) conjugate meningococcal vaccines (Menactra™ and Menveo™)8,9 have been licensed and made available in the United States of America (USA) and Menveo™ in the European Union (EU).10 Neisseria meningitidis is a bacterium that is only pathogenic in humans. It colonizes the nasopharynx and can be transmitted from person to person by direct contact with respiratory secretions or saliva, or via aerosol droplets.11 Adolescents and young adults constitute the greatest proportion of carriers.12–14

Meningococci have a polysaccharide capsule that protects them from desiccation and immune-mediated host defenses.15 Pili extend from the outer membrane and through the capsule to facilitate the initial attachment of meningococci to human cells and their movement over epithelial surfaces.

Classification of N. meningitidis into serogroups is based on the immunologic reactivity of their capsular polysaccharides; 13 serogroups have been identified: A, B, C, Y, W135, X, D, 29E, H, I, K, L and Z, although only the first six are involved in human pathology.16 Because the polysaccharides are the major meningococcal virulence factor (for instance polysaccharides are involved in the resistance of the bacterium against antimicrobial peptides and encapsulated wild-type N. meningitidis have substantially reduced adherence to dendritic cells compared with unencapsulated strains17–19) and because they are the outermost antigens on the surface of the bacterium, all licensed vaccines contain one or more A,C,W and Y polysaccharides.

Although the conjugate vaccines against meningitidis diseases have improved coverage for infants and adolescents, broadly effective meningococcal B vaccines are not yet available. This is a crucial fact, as Neisseria meningitidis group B (NMB) is now a predominant cause of the disease in industrialized countries.20,21 Recent research into N. meningitidis genomics and reverse vaccinology may be able to solve this seemingly intractable problem.22–27

This review highlights the evolution of conjugate meningococcal vaccines in general and discusses how this kind of vaccine can contribute to containing meningococcal disease.

Epidemiology

As for other bacteria, our knowledge of meningitis pathogenesis has rapidly expanded in recent decades. The capacity of N. meningitidis to colonize humans efficiently depends on its ability to evade the immune system. Indeed, its extensive variation of surface antigens, resistance to antimicrobial molecules, such as LL-37, resistance to complement, etc., are important mechanisms of survival for N. meningitidis in its sole niche of life.17 Transmission of N. meningitidis often leads to transient asymptomatic carriage. Carriers are the reservoir of the bacterium and, for this reason, may be regarded as the focus of infection control measures.28 Although carriage is relatively common, the majority of carriers do not acquire serious invasive disease. Only when N. meningitidis passes through the mucosa into the bloodstream and subsequently moves into the cerebrospinal fluid (CSF) can the patient develop invasive disease.29 This unfortunate event can be determined by a concurrence of different situations of risk, such as: asplenia, properdin deficiency, complement deficiency or depletion (C3, C5-9), previous or concomitant viral respiratory tract infection (influenza virus neuroaminidase could favor the adhesion of the bacterium to the surface of respiratory cells), chronic underlying disease, young age, crowded environmental conditions (e.g., discothèques, pubs, dormitories, etc.), living in close contact with others (e.g., students, military recruits, crowded homes), low socio-economic status, close contact with a patient or a carrier, travel to regions with a higher risk of meningococcal infection (e.g., African meningitis belt territories) and occupational exposure (e.g., healthcare or laboratory workers).29–31

The epidemiology of meningitis is very variable. In industrialized nations morbidity ranges from 0.2 to 4.7 cases per 100,000 inhabitants, whereas in sub-Saharan Africa and in the territories of the “meningitis belt” epidemics involve from 100 up to over 1,000 cases per 100,000 population.21,32

In the pre-antibiotic era, the meningitis case-fatality rate ranged from 70 to 90%; nowadays, it is estimated to be 7–14%. However, up to 20% of survivors suffer permanent sequelae. For example, a retrospective survey carried out in the province of Quebec, Canada, on 304 cases that occurred in 1990–94 reported a case-fatality rate of 14.0% and a percentage of sequelae of 15.3% (scarring 11.5%, amputation 4.6%, hearing loss 1.9% and renal failure 1.1%).15,33–35

From Polysaccharide Plain Vaccines to Conjugate Meningococcal Vaccines

Early attempts to develop vaccines against N. meningitidis were made from 1900 to 1940, using killed whole bacteria.36,37 Excessive reactogenicity, uncertainty as to the immunity conferred by these preparations, and the successes of treatment and prevention by means of antibiotics delayed the development of new vaccines. Subsequently, however, the phenomenon of sulfonamide-resistance38,39 prompted renewed interest in developing meningococcal vaccines. Indeed, at the end of the 1960s, Gotschlich et al. developed a new approach to purifying high molecular-weight meningococcal polysaccharides, which were thus rendered safe and immunogenic in humans.40,41 Meningococcal capsular polysaccharides purified in this manner are the basis of the currently licensed bivalent (A and C) and quadrivalent (A, C, Y and W-135) polysaccharide vaccines.

Plain meningococcal vaccines are licensed in bivalent (groups A and C [Meningivac™]) and quadrivalent (groups A, C, W135 and Y [Menomune™ and ACWYVax™]) formulations. GlaxoSmithKline Biologicals has developed a trivalent preparation (groups A, C and W-135) only for use in W135 epidemics that occur in sub-Saharan Africa.

Immunity elicited by meningococcal serogroups A, C, Y and W-135 is serogroup-specific. The efficacy of N. meningitidis serogroup A polysaccharide vaccine has proved to be 89–100% in clinical trials (army recruits and children aged three months to five years).42,43 Group A and C polysaccharide vaccines displayed 85% efficacy in clinical trials performed in settings in which meningococcal disease was epidemic.44 When polysaccharides Y and W-135 were used, they proved immunogenic only in subjects over two years of age.45,46 Meningococcal plain polysaccharide vaccines have shown a good safety profile, the most common adverse events being mild.47,48 Polysaccharide vaccines have yielded good results in army recruits. For instance, a bivalent vaccine for Italian army recruits introduced in 1987 (since 1991 tetravalent preparations have been used) displayed 90% efficacy,49 leading to a dramatic reduction in cases in the population of the Italian army.49

Because most cases of meningitis in Africa are sustained by serogroup A N. meningitidis, polysaccharide meningococcal vaccines have been used in several circumstances. In 2003, Robbins et al.50 suggested the use of meningococcal polysaccharide vaccine twice in infancy, followed by quadrivalent vaccine in children aged from two to six years. However, as underlined by Birmingham et al.51 this policy could be ineffective and possibly harmful. Indeed, many studies have described the short-lived immunity provided by group A meningococcal polysaccharide vaccine, its poor immunogenicity in young children, and the fact that multiple doses of group A meningococcal polysaccharide vaccine in childhood may actually attenuate the serum bactericidal antibody response to N. meningitidis. Indeed, the principal limitation of polysaccharide plain vaccines is that they do not stimulate the T- cell and so do not induce immunologic memory. Because of the above-mentioned deficiencies, meningococcal conjugate polysaccharide vaccines have been studied since the mid-1990s.52,53

Carbohydrate-Protein Conjugate Vaccines

In 1929, Avery and Goebel showed that conjugating a bacterial polysaccharide with a carrier protein elicited a stronger antibody response to the carbohydrate moiety.54 The discovery of T helper cells and their role in helping B cells to produce antibodies enabled antigens to be classified as T-cell-independent (TI) or T- cell-dependent (TD).55 With TD antigens, an immune response occurs at birth or shortly afterwards, affinity maturation of the B -cell response takes place, immunologic memory occurs and adjuvants can induce an enhanced response.56

TI antigens are divided into two groups. The first group comprises B-cell mitogens and includes polysaccharides. The second group comprises polysaccharides which have repeating epitopes. Antigens of the second group induce an immune response only 3–18 months after birth and infants (less than two years of age) generally have a poor response; IgM is mainly elicited and produced in the spleen57 and there is no affinity maturation of the antibody response, no immunologic memory and no enhancement of response by adjuvants.

Several immunogenic proteins have been studied, but only five have been used: diphtheria (D) or tetanus (T) toxoid, CRM 197 (a non-toxin variant of diphtheria toxin), OMP: a complex outer-membrane protein mixture derived from the B₁₁ strain of N. meningitidis serogroup B and protein D, derived from non-typeable Haemophilus influenzae (NTHi). Furthermore, the Pseudomonas aeruginosa recombinant exoprotein A (rEPA) appears to be another promising potential carrier protein. The polysaccharides or an oligosaccharide are linked to the carrier.

H. influenzae type b (Hib), the first conjugate vaccine to be prepared, was developed in 1992.58 Three preparations were licensed in the USA (conjugated with CRM 197, tetanus toxoid and OMPs, respectively). These vaccines are highly immunogenic in infants and immunocompromised persons, and they are 95–100% effective in eliminating all Hib disease when used in the routine infant vaccination schedule.59–63

Subsequently, pneumococcal conjugate vaccines were developed. Pneumococcal conjugate vaccines require the individual conjugation of each serotype with a carrier protein. There are 90 different serotypes, with different capsular polysaccharides, but only 7–13 are very important for human pathology. A heptavalent conjugate (using CRM 197 as a carrier) vaccine has been licensed in the USA and Europe,5 and another six polysaccharides have recently been added.7 Another conjugate pneumococcal vaccine that has recently been licensed in Europe is a 10-valent vaccine6 absorbed on aluminum phosphate and conjugated with tetanus toxoid, diphtheria toxoid and protein D derived from non-typeable Haemophilus influenzae. These preparations are highly protective against invasive disease and offers partial protection against other diseases, such as pneumonia and otitis media, caused by S. pneumoniae.64,65

Apart from meningococcal vaccines, which are the focus of this review and which are considered in greater detail in the following sections, other polysaccharide vaccines are under study, such as vaccines against: Salmonella typhi,66 Group B Streptococcus,67 Staphylococcus aureus,68 and Escherichia coli.69

Table 1 provides a summary of carbohydrate-protein conjugate vaccines and meningococcal conjugate vaccines.

Conjugate Vaccines against Meningococci

The diversity and molecular mimicry of N. meningitidis strains have been the main obstacle to developing meningococcal vaccines. The available anti-meningococci vaccines, which contain purified capsular polysaccharides A, C, W135 and Y, are not so effective in eliciting immunological memory or generating an effective immune response in infants.70,71 Conjugating capsular polysaccharides with carrier protein enabled these problems to be overcome, first with regard to N. meningitidis of group C and subsequently for groups A, W135 and Y. However, the very insidious mimicry of Neisseria of group B has thwarted any such solution for this type of bacteria. Recently, Novartis has developed a new vaccine containing five antigens; one of these, named fHBP (factor H Binding Protein), is contained in another new vaccine developed by Wyeth. These encouraging products are currently being assessed in phase II and III clinical trials.27

Meningococcal serogroup C conjugate vaccines were introduced for infants, toddlers and teenagers in the UK beginning in 1999.72 The licensed preparations are conjugated with CRM 197 protein or tetanus toxoid.

Before the MenC conjugate vaccine was licensed, the incidence of meningococcal disease in developed countries ranged from <1 to as high as 14.3 cases per 100,000 population, the proportion of serogroup C disease varying by country.73 After the introduction of the MenC conjugate vaccine in England and Wales, over 90% of infants aged <1 year and 85% of children/adolescents aged from 1 to 18 years were immunized under the initial campaign.74 The incidence of serogroup C disease subsequently fell dramatically.75 Indeed, the number of laboratory-confirmed meningitis C cases decreased from 954 in 1998/99 to 64 in 2003/04, and deaths attributable to N. meningitidis serogroup C fell from 118 (98/99) to 9 (2003/2004).75 By 2006, the MenC conjugate vaccine had been introduced into routine vaccination schedules across Europe, leading to a 10-fold drop in the incidence of meningococcal serogroup C disease.4

After the introduction of the Men C conjugate vaccine in the UK, the magnitude of the decrease in nasopharyngeal carriage was consistent with the acquisition of herd immunity, as demonstrated by the reduction in the number of cases among unvaccinated subjects.75,76 Furthermore, among 15- to 19-year-old British students, the state of carriage of meningococci encoding disease-associated capsule, Sia Dc, decreased from 5.88% of isolated Neisseriae in 1999 to 2.68% in 2001. It was also evident that, although the prevalence of serogroup C carriage was significantly reduced, there was no serogroup replacement.76 Indeed, the prevalence of carriage of the meningococci most commonly associated with serogroup B disease, SiaD3 members of the ST-41/44 complex, was unaffected by the introduction of the vaccine.76 However, while the antibody response is sustained in adolescents following vaccination with serogroup C meningococcal conjugate vaccine (MenC), in young children it rapidly wanes,77 and one or two booster doses appear to be required in subjects aged 6–12 years.

In 2010, Ray Borrow et al.78 published the results of a study on the kinetics of antibody persistence following administration of a combination of meningococcal serogroup C and Haemophilus influenzae type b conjugate vaccine in healthy British infants primed with a monovalent meningococcal serogroup C conjugate vaccine.

These results demonstrated:

• The limitations of the serum bactericidal antibody assay (SBA) for the assessment of protective meningococcal immunity, and the difficulties of interpreting and comparing the results obtained in different studies;

• The usefulness of a booster dose of MenC, possibly combined with Haemophilus influenzae polysaccharide conjugated vaccine, at 12–15 months of age;

• The superiority of de-O-acetylated C polysaccharide conjugated with tetanus toxoid (Neis-Vac-C™) over O-acetylated C polysaccharide conjugated with CRM 197 (Menjugate™ and Meningitec™);

• However, the most important finding of Borrow's study was that the antibody declined to below the pre-booster level 24 months after booster administration; this indicates the possible need for a booster dose for schoolchildren and adolescents.

Subsequently, after the enlarged campaigns of MenC vaccination, because of the relatively high incidence of group Y disease in the USA (which accounted for 37% of all cases of meningococcal disease between 1997 and 2002),27 a quadrivalent A, C, W-135 and Y conjugate (diphtheria toxoid as a carrier) vaccine (MCV4) was licensed in the USA (Menactra™) in 2005 for subjects aged 2–55 years.

The immunogenicity of MCV4 has been compared with that of a plain tetravalent A, C, W-135 and Y meningococcal vaccine (MPSV4, Menomune™) in 11- to 18-year-old subjects.79 In that study, immunogenicity was assessed by means of a serum bactericidal assay, using baby rabbit serum as a source of complement (rSBA). A ≥4-fold increase in rSBA was assumed to indicate effectiveness of the vaccine.80 The percentages of participants with at least a four-fold increase in rSBA level 28 days after MCV4 administration were: 96.7%, 92.7%, 91.7% and 81.8% for serogroups W-135, A, C and Y, respectively. Another comparative study on the immunogenicity of plain versus conjugate vaccine was carried out in subjects aged 18–55 years; the percentages of volunteers with at least a four-fold increase in rSBA 28 days after vaccination with the conjugate vaccine were: 89.4%, 80.5%, 88.5% and 73.5% for serogroups W-135, A, C and Y, respectively.81 A randomized clinical trial involving children aged 2–10 years was performed by Pichichero M, et al. in 2005.82 The percentage of those seronegative at the baseline who developed a ≥4-fold increase in rSBA 28 days after vaccination was significantly (p < 0.05) higher in the MCV4 group than in the MPSV4 groups for serogroups A, C, Y and W-135. However, while this vaccine is effective in adolescents and young adults, its immunogenicity is poor in infants.27 Indeed, in infants aged 2, 4 and 6 months who received three doses of MCV4, the serum bactericidal antibody responses one month after the first dose were low.83 In those who received 4 Mg of each polysaccharide/per dose, the GMTs of rSBA were about 1:20 for C, Y and W-135 polysaccharides. These titers were about 50-fold lower than those obtained after three doses of meningococcal C conjugate vaccine administered at 2, 3 and 4 months of age. Furthermore, a study performed in toddlers (11–23 months) who received two doses of MCV4 two months apart showed that they developed high GMTs of 1:300 to 1:400 against the polysaccharides C, Y and W-135 and of 1:3,000 against polysaccharide A.84

In general, as expected, the immune response after MCV4 administration is age-related, declining as the age of immunization decreases. The fact that MCV4 is poorly immunogenic in infants explains why the vaccine has not been licensed for subjects under two years of age. This only apparently goes against the dogma of conjugate vaccines. Indeed, as argued by Adam Finn85 with regard to the antibody response to PS antigens: “It appears that a distinct signaling pathway may regulate this important antibody response pathway, whose size and character vary not only between individuals and with age, but also with previous exposure and between antigens.” However, meningococcal conjugate vaccines remain an important improvement in the field of meningococcal disease prevention, even though further knowledge is needed in order to enhance the performances of these preparations.

Regarding the safety of Menactra™, randomized trials have shown that the vaccine is generally well tolerated and has a good safety profile: similar to that of plain vaccine. Serious adverse events (SAE) following conjugate vaccine administration have been uncommon. Fever has been reported more frequently among MCV4 recipients aged 11–18 years or 18–55 years than among plain vaccine recipients.79,82 From 2005 to 2008, >15 million doses of Menactra™ were distributed. In this period, the passive US Vaccine Adverse Event Reporting System (VAERS) received 26 reports of confirmed Guillain-Barré Syndrome (GBS), which occurred six weeks after administration of the vaccine.86 However, the US Center for Disease Control and Prevention (CDC) deemed these data insufficient to establish an increased risk of GBS for recipients of MCV4, and recommended continuation of the already defined strategy of vaccination. Only a family history of GBS was regarded as a relative contraindication for MCV4.87

Other multivalent vaccines are now being studied in which CRM 197 protein or tetanus toxoid is used as a carrier. Specifically, CRM 197 has been used by Novartis to prepare a tetravalent conjugated meningococcal vaccine (MenACWY). This vaccine, named Menveo™, was studied in infants by Snape M, et al. in 2008.88 These authors studied three protocol regimens: administration at 2-, 3- and 4-months (UK234 group), at 2, 4 and 6 months (CA246 group), and at 2- and 4-months (UK24 group). The proportions of recipients in the first group who had hSBA (serum bactericidal assay, using human serum as a source of complement) titers ≥1:4 after primary immunization were: serogroup A, 93% (95% CI: 84–98%); C, 96% (89–99%); W135, 97% (90–100%); and Y, 94% (86–98%). For the per-protocol analysis of 2, 4 and 6 month-old MenACWY recipients, the percentages (95% CI) of responders were: A, 81% (71–89%); C, 98% (92–100%); W135, 99% (93–100%); and Y, 98% (92– 100%). At least 84% of 2 and 4 month-old MenACWY recipients displayed hSBA titers ≥1:4 for serogroups C, W-135 and Y after primary immunization, as did at least 60% for serogroup A. All subjects (UK234, CA246 and UK24 groups) received a booster dose of conjugate vaccine at 12 months and had a strong secondary response, also for serogroup A, for which the percentages of protected subjects were seen to have declined markedly by month 12. The percentage of participants experiencing intense local tenderness after one or more administrations was 7% in the UK234 group and 10% in the CA246 group; however, no more than 1% experienced intense erythema or induration. A temperature of 40°C or more was observed within a week of MenACWY administration in only one participant. The authors concluded that MenACWY was well tolerated and immunogenic in infancy.

In another clinical trial, Black et al.89 studied the MenACWY-CRM vaccine in children aged 2–10 years who received the conjugate vaccine or a licensed quadrivalent vaccine (MPSV4). For all serogroups, more MenACWY-CRM recipients had achieved an hSBA (human Serum Bactericidal Antibody) titer ≥1:4 by month 1 post-vaccination (A: 82%; C: 83%; W-135: 95%; and Y: 91%) than recipients of the plain vaccine MPSV4 (A: 45%; C: 66%; W-135: 71%; Y: 91%). Both vaccines were well tolerated.

In 2009, Jackson LA et al.90 published the results of a phase III clinical trial carried out in adolescents (11–18 years of age) with MenACWY-CRM vaccine. In their study, this vaccine was compared with another tetravalent conjugate vaccine, Menactra™(MCV4). The hSBA geometric mean titers after MenACWY-CRM vaccination were higher than the hSBA geometric mean titers after Menactra™ vaccination and criteria for superiority were met for this end-point for all 4 serogroups. Reactogenicity was similar, with 64% of the Menveo™ recipients and 70% of the Menactra™ recipients reporting mild and/or moderate solicited reactions. Neither vaccine was associated with serious adverse events.

Gill CJ et al.91 reported the results of vaccination with Menveo™ in 19–65 year-old subjects randomized to receive the MenACWY-CRM197 vaccine, MCV4 (Menactra™; 19–55 yearolds [study 1]) or MPSV4 (56–65-year-olds [study 2]). Overall, 4,190 adults were recruited. Menveo™ met non-inferiority criteria versus MCV4 for all immunogenicity end-points for all serogroups. Menveo™ was statistically superior to MCV4 in study 1 for serogroups Y (all end-points) and W135 (sero-response and hSBA titer ≥1:8). MenACWY-CRM 197 was statistically superior to MPSV4 for all end-points for serogroups C, W-135 and Y. In 56–65 year-olds, Menveo™ induced GMTs from 1.4- to 5-fold higher than did MPSV4.

In April 2010, Gasparini et al.92 published the results of a phase III clinical trial on the administration of MenACWY-CRM vaccine in adolescents and young adults. This study evaluated the safety, tolerability and immunogenicity of the vaccine when administered concomitantly with a combined tetanus, reduced diphtheria and acellular pertussis (dTaP) vaccine, in subjects aged 11–25 years. Subjects received either MenACWY-CRM and dTaP, MenACWY-CRM and saline placebo, or dTaP and saline placebo. No significant increase in reactogenicity and no clinically significant vaccine-related AEs occurred when MenACWY-CRM and dTaP were administered concomitantly. Similar immunogenic responses to diphtheria, tetanus and meningococcal (serogroups A, C, W-135 and Y) antigens were observed, regardless of concomitant vaccine administration. Anti-pertussis antibody responses were comparable between the vaccine groups for filamentous hemagglutinin and slightly lower, although the difference was not clinically significant, for pertussis toxoid and pertactin when the two vaccines were administered concomitantly. These results indicate that the investigational MenACWY-CRM vaccine is well tolerated and immunogenic and can be co-administered with dTaP in adolescents and young adults. Menveo™ can also be safely administered concomitantly with a quadrivalent HPV vaccine (Gardasil™). Concomitant administration does not impact on the level of immune response (in terms of seroprotection).93

Another meningococcal conjugate quadrivalent vaccine, MenACWY-TT, is being developed by GlaxoSmithKline.

Ostergaard et al.94 recently published the results of two studies carried out from 2003 to 2006 on a tetravalent A, C, W-135 and Y vaccine conjugated with tetanus toxoid (MenACWY-TT). One hundred seventy-five healthy subjects aged 15–25 years were recruited. In the first study, subjects were randomly allocated to two equal groups to receive a single dose of one MenACWY-TT formulation or the comparator MenACWYPS vaccine. In the second study, subjects were stratified by age (15–17 and 18–19 years) and subdivided into five study groups to receive a single dose of four other MenACWY-TT formulations or the MenACWYPS vaccine. The proportion of rSBA responders to each serogroup (A, C, W-135 and Y) did not statistically differ between the comparator and each of the five formulations, except for serogroup A, which was lower after administration of one of the MenACWY-TT formulations. In Study 1 subjects, who were followed up for three years, bactericidal antibody persistence was similar to that seen with the licensed ACWY polysaccharide vaccine for MenA and MenC and higher for MenW-135 and MenY. Furthermore, MenACWY-TT vaccine formulations were well tolerated in adolescents and young adults.

A comparative study of MenACWY-TT vaccine versus MenC-CRM197 was performed by Knuf et al.95 in children aged 2–10 years. One month after vaccination, the MenACWY-TT vaccine induced a robust immune response for serogroups A, C, W-135 and Y. However, GMT for serogroup C was significantly lower with MenACWY-TT than with MenC-CRM197. MenACWY-TT also showed an acceptable safety profile.

Maurer et al.96 studied MenACWY-TT co-administered with DTPA-HBV-IPV/HIB in comparison with DTPA-HBV-IPV/HIB administered alone. Co-administration of MenACWY-TT with DTPA-HBV-IPV/HIB was non-inferior to DTPA-HBV-IPV/ HIB alone in terms of immunogenicity of all DTPA-HBV-IPV/HIB vaccine components in young children (12–23 months). Co-administration was well tolerated in this study population.

Vesikari et al.97 studied the co-administration of MenACWY-TT with MMRV in healthy toddlers aged 12–13 months in Finland. Co-administration was well tolerated and proved to be non-inferior to administration of either vaccine alone in terms of immunogenicity.

Table 2 provides a summary of the studies of meningococcal conjugate vaccines.

Meningococcal Group A Conjugate Vaccine for Use in Africa

Although meningococcal diseases occur throughout the world, the attack rates in the Saharan and Sub-Saharan regions of Africa are many times higher than those seen in any other part of the world. Indeed, in 2009, over 70,000 meningitis cases and 3,200 deaths were reported in Nigeria, Niger and Chad alone.98 Cases of meningitis in the “meningitis belt” regions are usually caused by serogroup A N. meningitidis. In the decade 1996–2006, more than 700,000 cases were reported.99

Because plain polysaccharide vaccines are not very effective in protecting young children, do not create long-lasting immunity and do not confer a “herd effect,” in 2001 a partnership was created between the WHO (World Health Organization) and PATH (Program for Appropriate Technology in Health) to eliminate epidemic meningitis in Africa through the development of an affordable meningococcal A conjugate vaccine (MenA). However, as it became clear that no manufacturer in the developed world could produce a Men A conjugate vaccine at less than US$0.50 per dose, an Indian manufacturer, the Serum Institute of India Ltd. (SIIL), undertook to produce an appropriate affordable MenA vaccine. The meningitis A polysaccharide was conjugated to Tetanus Toxoid and toxicology and animal studies were successfully completed. Indeed, the animal studies suggested that the conjugate vaccine was highly immunogenic.99,100

In 2004, partnerships were set up with laboratories at the US CDC and the UK Health Protection Agency (HPA) in order to conduct serological testing for the clinical trials. The following year, the first clinical trial, PsA-TT-001, was launched. This phase 1 clinical trial evaluated the safety and immunogenicity of MenA in 74 healthy Indian adults and showed that the vaccine was safe and immunogenic.101 Consequently, in 2006 the pivotal phase 2 trial started (PsATT-002). The younger age-groups targeted by the mass vaccination campaign were considered in this study.100

In 2007, a severe epidemic occurred in Burkina Faso (26,878 cases were reported) and the government made a plea for international assistance. SIIL and the MPV (Meningitis Vaccine Project) developed a strategic plan for vaccine licensure. Meanwhile, results from PsA-TT-002 confirmed that the vaccine was safe and highly immunogenic. As a result, Phase 2/3 studies were launched. Other phase 3 and 2 studies started in 2008 (e.g., in Ghana).100,102,103

In 2009, SIIL submitted the MenA vaccine regulatory file to the Drugs Controller General of India (DCGI) and to the WHO for prequalification. In December 2009, the Regional (Maharahstra State) Food and Drug Administration granted marketing authorization, enabling the MenA conjugate vaccine (MenAfriVac™) to be exported and used in Africa.100 Thus, an affordable (US$0.40 per dose), safe and highly immunogenic vaccine is now practically available to combat meningitis in Africa.104 Adequate resources are now required in order to offer MenAfriVac™ on a large scale in all the countries of the meningitis belt.

Recommendations of Meningococcal Conjugate Vaccines

Meningococcal serogroup C conjugate vaccines (MenC) were introduced into the UK childhood vaccination program in late 1999,105 and scheduled for all subjects under the age of 18 years. In 2002, these vaccines were also made available to those aged 20–24 years. Since protection declines over time, especially when the vaccines are administered before one year of age,3 booster doses are now offered in the UK, especially to subjects at high risk.106

In January 2005, the MCV4 vaccine was licensed for use in subjects aged 11–55 years. In the same year, the Advisory Committee on Immunization Practices (ACIP) recommended routine vaccination of young adolescents (defined as persons aged 11–12 years) with MCV4.107 In 2009, the ACIP updated the 2005 recommendations and now recommends quadrivalent meningococcal conjugate vaccine for all persons aged 11–18 years and for persons aged 2–55 years at increased risk of meningococcal disease.108 Populations at increased risk are: college freshmen living in dormitories;109,110 microbiologists who are routinely exposed to isolates of N. meningitidis;111 military recruits;112 persons who travel to or reside in countries in which N. meningitidis is hyperendemic or epidemic;113 individuals who have persistent complement deficiencies (e.g., C3, properdin, Factor D and late complement deficiencies);114 subjects who have anatomic or functional asplenia.115

In February 2010, the Food and Drug Administration (FDA) licensed a new quadrivalent meningococcal MenACWY-CRM (Menveo™) vaccine. Menveo™ has been licensed for use in a single dose in persons aged 11–55 years.116

In March, 2010 the EMEA (European Medicines Agency) licensed MenACWY-CRM (Menveo™) vaccine for use in subjects aged over 11 years. Menveo™ is recommended for teenagers and adults at risk in some European countries.

The Canadian National Advisory Committee on Immunization (NACI) has made recommendations on the use of meningococcal C conjugate vaccine for infants (i.e., children <1 year of age), children from one to four years of age, adolescents and young adults.117 The vaccine should be considered for children ≥5 years of age who have not reached adolescence. In an updated statement on meningococcal C conjugate vaccines in 2005,118 the NACI recommended a three-dose schedule for young infants using Menjugate^® (Novartis Vaccines) or Meningitec™ (Wyeth Canada). The first dose was to be given no earlier than two months of age and the other doses were to be separated by at least one month; at least one dose in the primary series was to be given after five months of age. For infants aged four to eleven months, a two-dose schedule was recommended for Menjugate^® and Meningitec™ with the doses given at least one month apart. In the 2005 statement, the recommended schedule for NeisVac-C^® (GlaxoSmithKline) was changed to two doses, the first being given no earlier than two months of age and the second at least two months later. The NACI recommended that one dose of the primary series be given after five months of age. Children ≥1 year of age require only one dose, whichever of the three meningococcal C conjugate products is used.

In Italy, anti-meningococcal C-conjugate vaccine (MenC) has so far been recommended for use in specific groups. Since the health system is decentralized, the Regional Health Authorities (RHAs) can decide to recommend vaccination for other target populations. Recommendations regarding MenC have been made in 19 out of 20 Italian regions. Free-of-charge MenC vaccinations targeting all infants have been recommended in nine regions.119

Conclusions

Carbohydrate-protein conjugate vaccines constitute a good approach to solving the problems created by the ability of several bacterial pathogens to circumvent human defenses. While it sometimes takes a long time to develop live attenuated vaccines, the development of conjugate vaccines has taken place over two decades. The impetus to prepare meningococcal vaccines is justified by the fact that meningococcal disease is a serious health threat. In view of the rapid onset, uncharacteristic symptoms and high fatality rate even after antibiotic treatment, the prevention of life-threatening meningococcal illness is a priority.

Meningococcus is a highly adapted and successful commensal that has developed several mechanisms of survival in intimate contact with the complex array of microbial effector tools of the human immune system. However, there are now very real prospects of obtaining an effective meningococcal vaccine with a broad spectrum of action, irrespective of the capsular serogroup. Indeed, there is evidence that new protein vaccines against N. meningitidis group B may also provide cross-protection against capsular group A, C, W-135 and Y strains.120

At present, there are two licensed meningococcal tetravalent conjugate vaccines that display considerable potential to improve the control and prevention of a severe disease that evokes great fear in health-care workers and parents. It will be very important to adopt a correct policy of vaccination, taking into account the epidemiological characteristics of the disease and considering that the principal goal of the vaccination strategy is to reduce carriage rates by creating solid herd immunity. A good policy of anti-meningococcal vaccination must take into account:

• the decrease in maternal bactericidal antibody after birth, which reaches its nadir between 6 and 24 months;121

• the problem of immaturity of the immune system in infants, which is only partially solved by the conjugation of polysaccharides;

• the recent re-analysis of the epidemiology in the USA, which reveals that, although the incidence of disease is nearly nine times higher for children aged <1 year than in the general US population, it only represents about 9% of all vaccine-preventable cases and 2% of all preventable deaths;122,123

• the fact that 60% of cases and 70% of fatalities in the USA occur in adults aged 25–64 years;

• the non-negligible number of cases throughout childhood;

• the studies by Caroline Trotter, Ray Borrow, Elizabeth Miller, Nick Andrews, et al. on the UK experience of meningococcal C conjugate vaccine and the studies by de Wals on the mathematical and economic models of the impact of MenC vaccination in different scenarios;124

• the behavior of immunity in different age-classes in the MenC pre-vaccine era.125,126

Therefore, the best targets of meningococcal quadrivalent conjugate vaccines appear to be: infants at 3 and 12 months of age, children at 6 years of age and adolescents at 12 years of age. This combined strategy could rapidly create effective herd immunity and reduce the incidence of the disease in all age-classes, as happened in the UK after the enlarged Men C vaccination campaign.

Abbreviations

ACIP	=	advisory committee on immunization practices
AE	=	adverse event
CDC	=	US Center for Disease Control and Prevention
CRM 197	=	a nontoxic variant of diphtheria toxin
CSF	=	cerebrospinal fluid
DT	=	diphtheria toxoid
dTaP	=	diphtheria (dose for adult), tetanus and acellular pertussis vaccine
DTaP	=	diphtheria (dose for children), tetanus and acellular pertussis vaccine
EMEA or EMA	=	European Medicines Agency
EU	=	European Union
FDA	=	Food and Drug Administration
fH	=	factor H
fHBP	=	factor H Binding protein
GAVI	=	global alliance for vaccines and immunization
GBS	=	Guillain-Barrè syndrome
GMT	=	geometric mean titer
Hib	=	Haemophilus influenzae type b
HPA	=	UK Health Protection Agency
hSBA	=	serum bactericidal antibody assay with human complement
MC-4	=	quadrivalent A,C,W-135 and Y conjugate (dipheria toxoid as carrier) vaccine, usually used for Menactra™
MCC	=	meningococcal C conjugate vaccine
MCV4	=	quadrivalent A,C,W-135 and Y conjugate (diphtheria toxoid as carrier) vaccine
Men A-TT	=	meningococcal A tetanus toxoid conjugate vaccine
MenA	=	meningococcal A conjugate vaccine
MenACWY-CRM197	=	quadrivalent A,C,W-135 and Y conjugate (CRM 197 as carrier) vaccine
MenACWY-TT	=	quadrivalent A,C,W-135 and Y vaccine conjugated with tetanus toxoid
MenC	=	meningococcal C conjugate vaccine
MLTS	=	multilocus sequence typing
Mn A	=	group A meningitis
MPS-4	=	plain tetravalent A,C,W-135 and Y meningococcal vaccine
MPSV4	=	plain tetravalent A,C,W-135 and Y meningococcal vaccine
MVP	=	meningitis vaccine project
NACI	=	Canadian National Advisory Committee on Immunization
NMB	=	Neisseria meningitidis group B
OMP	=	complex outer-membrane protein mixture derived from N. meningitidis
PATH	=	program for appropriate technology in health
PS	=	polysaccharide
PsA-TT	=	meningococcal A tetanus toxoid conjugate vaccine
rSBA	=	serum bactericidal antibody assay with rabbit complement
SAE	=	serious adverse events
SBA	=	serum bactericidal antibody assay
siaDb or siaDB	=	polysialyltransferase gene (siaD) for serogroup B N. meningitidis
siaDc or siaDC	=	polysialyltransferase gene (siaD) for serogroup C N. meningitidis
SIIL	=	Serum Institute of India Ltd.
ST-11	=	sequence type-11 complex
TD	=	T cell-dependent
TI	=	T cell-independent
TT	=	tetanus toxoid
UK	=	United Kingdom
USA	=	United States of America
VAERS	=	vaccine adverse event reporting system
VFs	=	virulence factors
WHO	=	World Health Organization

Figures and Tables

Table 1 A summary of carbohydrate-protein conjugate vaccines and meningococcal conjugate vaccines

Formulation, vaccine	Reference(s)	Clinical status	Results
Hib type b polysaccharide conjugated with CRM 197* Hib type b polysaccharide conjugated with CRM 197* Hib type b polysaccharide conjugated with tetanus toxoid Hib type b polysaccharide conjugated with tetanus toxoid Hib type b polysaccharide conjugated with OMP**	59–63	Available licensed (HibTiter™)	These vaccines are highly immunogenic in infants and in immunocompromised persons, and are 95–100% effective in eliminating all Hib diseases when used in the routine infant vaccination schedule.
		Available licensed (Vaxem Hib™)
		Available licensed (Acthib™)
		Available licensed (Hiberix™)
		Available licensed (PedvaxHIB™)
Hib type b polysaccharide conjugated tetanus toxoid with + MenC polysaccharide conjugated with tetanus toxoid	77	Available licensed (Menitorix™)	Administered at 6 years of age, elicited a very high antibody response in children who had received conjugate MenC in infancy.
S. pneumoniae 7 polysaccharides Conjugated with CRM197 S. pneumoniae 13 polysaccharides Conjugated with CRM197 S. pneumoniae 10 polysaccharides Conjugated with protein D (derived from non-typeable Hib), tetanus toxoid and diphtheria toxoid. Other S. pneumoniae polysaccharide vaccines have been licensed in the past, such as 8-valent, 9-valent and 11-valent vaccines.	5–7	Available licensed (Prevenar™)	These vaccines confer very high protection against invasive disease, and partial protection against other diseases, such as pneumoniae and otitis media. The replacement phenomenon was observed after wide use of the heptavalent vaccine.
		Available licensed (Prevenar-13™)
		Available licensed (Synflorix™)
Men C conjugated with CRM197 Men C conjugated with CRM197 Men C conjugated with tetanus toxoid Men C conjugated with CRM197	4, 72–78	Available licensed (Menjugate™) Available licensed (Meningitec™) Available licensed (Neis-vac™) Available licensed (Meninvact™)	After the introduction of Men C conjugate vaccine, the incidence of invasive disease fell dramatically; a decrease in nasopharyngeal carriage was also evident. However, while antibody response was sustained in adolescents, it rapidly waned in young children. This suggests the need for a booster dose for 6- and 12-year-olds.
Men A, C, W-135 and Y polysaccharide conjugated with diphtheria toxoid Men A, C, W-135 and Y polysaccharide conjugated with CRM-197	79–87 88–93	Available licensed (Menactra™) Available licensed (Menveo™)	Menactra™ is effective in adolescents and young adults but its immunogenicity is poor in infants. MenACWY-CRM has a good immunogenic power in children, young adults and subjects up to 65 years of age. Furthermore, it is possible to co-administer MenACWY-CRM with dTaP and HPV vaccines.
Men A polysaccharide conjugated with Tetanus Toxoid	100–105	WHO prequalification obtained; Regional (Maharashstra State—India) FDA authorization obtained (MenAfriVac™); Regulatory file submitted to Drug Controller General of India (DCGI)	The vaccine has proved safe and highly immunogenic in clinical trials on animals, adults, infants, young children and subjects aged up to 29 years.

* a non-toxin variant of diphtheria toxin.

** a complex outer-membrane protein mixture derived from N. meningitidis.

Table 2 Summary of studies of meningococcal conjugate vaccines

Location	Reference	Vaccine	Age-group	Study design	Safety	Immune response conclusions
UK	72–75	MenC	2 mo–18 yr	Vaccination campaign	Safe and well tolerated	The incidence of serogroup disease fell dramatically.
UK	75–76	MenC		Reduction in carriage of MenC	Safe and well tolerated	The reduction in carriage was considerable. No replacement occurred.
UK	74	MenC		Evaluation of long-term protection	Safe and well tolerated	Protection was very poor 1 year after administration.
UK	77	Hib-MenC	6–12 yr	Usefulness of administration of booster dose to 6–12-year-old children	Safe and well tolerated	Very high secondary response after booster dose administered one month later; 100% of participants had SBA >1:8 and 99.6% at one year.
Europe	4	MenC	2 mo–18 yr	Vaccination campaign	Safe and well tolerated	By 2006, the MenC conjugate vaccine had been introduced into routine vaccination schedules across Europe, and a 10-fold drop in the incidence of meningococcal serogroup disease was observed.
USA	79	MCV4	adolescents	MCV4 vs. Plain polysaccharide vaccine	Well tolerated with similar safety profile.	The percentages of participants with at least a 4-fold increase in rSBA level 28 days after MCV4 administration were: 96.7%, 92.7%, 91.7% and 81.8% for serogroups W-135, A, C and Y, respectively.
USA	81	MCV4	18–55-year-old subjects	Comparison of adjuvated vs. plain vaccine	Two vaccines were well tolerated and had similar safety profiles.	The proportion of volunteers with at least a 4-fold increase in rSBA level 28 days after vaccination with conjugate vaccine was: 89.4%, 80.5%, 88.5% and 73.5% for serogroups W-135, A, C and Y, respectively.
USA	82	MCV4	2–10 years	Comparison of adjuvated vs. plain vaccine	Two vaccines were well tolerated and had similar safety profiles.	The percentage of subjects seronegative at the baseline who developed a ≥4-fold increase in rSBA level 28 days after vaccination was significantly (p < 0.05) higher in the MCV4 group than in the MPSV4 group for serogroups A, C, Y and W-135.
USA	83	MCV4	6–13 weeks of age	Immunogenicity study of safety of 4 dosages of MCV4	Safe and well tolerated	The various dosages of vaccine showed poor immunogenicity.
UK, Canada	88	MenACWY-CRM197	Infants	To determine the immunogenicity of MenACWY-CRM197 three protocol regimens were studied: 1) administration at 2,3, and 4 months; 2) administration at 2,3, and 6 months; 3) administration at 2 and 4 months.	Safe and well tolerated	In the first group, the percentages of recipients with hSBA (serum bactericidal assay, using human serum as a source of complement) titers ≥1:4 after primary immunization were: serogroup A, 93% (95% CI: 84–98%); C, 96% (89–99%); W135, 97% (90–100%); Y, 94% (86–98%). In the per-protocol analysis of MenACWY 2-, 4- and 6-month recipients, the percentages (95% CIs) of responders were: A, 81% (71–89%); C, 98% (92–100%); W135, 99% (93–100%); and Y, 98% (92–100%). At least 84% of MenACWY 2- and 4-month recipients achieved hSBA titers ≥1:4 for serogroups C, W-135, and Y after primary immunization, as did at least 60% for serogroup A.
USA	89	ACWY-CRM197	2–10 years	CRM-197 and MPSV4	Both vaccines were well tolerated	For all serogroups, more menACWY-CRM recipients achieved an hSBA titer ≥1:4 at month 1 post-vaccination (A: 82%; C: 83%, W-135: 95%; Y: 91%) than recipients of the plain vaccine MPSV4 (A: 45%, C: 66%; W-135: 71%; Y: 91%).
USA	90	MenACWY-CRM197	11–18-year-olds	Comparison between MenACWY-CRM-197 and MCV4	Similar reactogenicity	The hSBA geometric mean titers after MenACWY-CRM vaccination were higher than after MCV4 administration.
USA	91	MenACWY-CRM197	19–65-year-old subjects subdivided in study 1 [19–55 years] and study 2 [56–65 years]	Comparison between MenACWY-CRM-197 and MCV4	Both vaccines were well tolerated	Menveo™ met non-inferiority criteria for all immunogenicity end-points for all serogroups versus MCV4. Menveo™ was statistically superior to MCV4 in study 1 for serogroup Y (all endpoints) and W135 (seroresponse and hSBA titer ≥1:8). MenACWY-CRM 197 was statistically superior to MPSV4 for all end-points for serogroups C, W-135 and Y in 56–65-years-olds.
Italy	92	MenACWY-CRM197	11–25 years	This study evaluated the safety, tolerability and immunogenicity of the vaccine when administered concomitantly with a combined tetanus, reduced diphtheria and acellular pertussis (dTaP) vaccine	No significant increase in reactogenicity on concomitant administration.	The results indicate that the investigational MenACWY-CRM vaccine was well tolerated and immunogenic, and can be co-administered with dTaP in adolescents and young adults.
USA	93	MenACWY-CRM197	11–18 years	Co-administration with quadrivalent HPV vaccine.	Safe and well tolerated	Menveo can be safely administered concomitantly with quadrivalent HPV vaccine (Gardasil™). Concomitant administration did not influence the level of immune response.
Belgium, Denmark	94	MenACWY-TT	15–25 years	Study 1: 2 groups received a single dose of one MenACWY-TT formulation or the comparator MenACWYPS vaccine. Study 2: participants were stratified by age (15–17 and 18–19 years) and subdivided into 5 study groups to receive a single dose of four other MenACWY-TT formulations or MenACWYPS vaccine.	The formulations were well tolerated in adolescents and young adults	In Study 1 subjects, who were followed up for 3 years, bactericidal antibody persistence was similar to that seen with the licensed ACWY polysaccharide vaccine for MenA and MenC and higher than for MenW-135 and MenY.
Germany, France	95	MenACWY-TT	2–10 years	A comparative study of MenACWY-TT vaccine versus MenC-CRM197	Acceptable safety profile.	One month after vaccination, the MenACWY-TT vaccine induced a robust immune response for serogroups A, C W-135 and Y. However, GMT for serogroup C was significantly lower with MenACWY-TT than with MenC-CRM197 vaccine.
Austria, Germany, Greece	96	MenACWY-TT	young children (12–23 months)	MenACWY-TT vaccine administered contemporarily with DTPA-HBV-IPV/HIB vaccine in comparison with DTPA-HBV-IPV/HIB vaccine administered alone.	Co-administration was well tolerated.	Co-administration of MenACWY-TT with DTPA-HBV-IPV/HIB was non-inferior to DTPA-HBV-IPV/HIB alone, in terms of the immunogenicity of all DTPA-HBV-IPV/HIB vaccine components.
Finland	97	MenACWY-TT	Healthy toddlers aged 12–13 months	Co-administration of MenACWY-TT vaccine with MMRV vaccine	Co-administration was well tolerated.	Co-administration of MenACWY-TT with MMRV vaccine was non-inferior to administration of either vaccine alone in terms of immunogenicity.
India	101	MenA conjugated with tetanus toxoid	Adults	To determine safety and immunogenicity of MenA	Safe and well tolerated	The vaccine was highly immunogenic.
Africa (Mali, Gambia, Ghana, etc.,)	102	MenA conjugated with tetanus toxoid	Infants, toddlers, and subjects up to 29 years	To determine safety and immunogenicity of MenA	Safe and well tolerated	The vaccine was highly immunogenic, and in 12- to 23-month-olds, the vaccine produced antibody levels 20 times higher than those obtained with the marketed polysaccharide (unconjugated) vaccine.