Development and characterisation of outer membrane vesicle vaccines against serogroup A Neisseria meningitidis

Neisseria meningitidis bacteria of serogroup A are causing recurring meningitis epidemics on the African continent. An outer membrane vesicle (OMV) vaccine against serogroup A meningococci made from a subgroup III serogroup A meningococcal strain was previously shown to induce antibodies with serum bactericidal activity (SBA) in mice. We have here further investigated the properties of OMV vaccines made from five different subgroup III serogroup A meningococcal strains grown in a synthetic medium with low iron content. In addition to the major outer membrane proteins (PorA, PorB, RmpM, Opa and OpcA), small amounts of the NadA, TdfH, Omp85, FetA, FbpA and NspA outer membrane proteins, as well as lipooligosaccharides, were detected in the vaccines. The OMV vaccines were used to immunise mice. Anti-meningococcal IgG antibodies in the mouse sera were analysed by immunoblotting and by enzyme-linked immunosorbent assay against OMVs, and against live meningococcal cells in SBA and a flow-cytometric assay. The vaccines induced antibodies with high SBA and opsonophagocytic activity. The strongest IgG responses were directed against PorA. Significant SBA responses were also observed against a subgroup III strain, which did not express PorA, whereas no SBA was observed against a clone IV-1 serogroup A strain. An OMV vaccine from serogroup A meningococci may be an alternative to polysaccharide and conjugate polysaccharide vaccines for Africa.     

The concept of "tailor-made", protein-based, outer membrane vesicle vaccines against meningococcal disease

Protein-based, outer membrane vesicle (OMV) vaccines have previously proven to be efficacious against serogroup B meningococcal disease in Norway and Cuba. Currently, a public health intervention is going on in order to control a serogroup B epidemic in New Zealand. The scale-up and standardization of vaccine production required for controlling the New Zealand epidemic has allowed the establishment of large-scale GMP manufacturing for OMV vaccines. The outcome of this will be licensing of the vaccine in New Zealand and possibly other countries. The availability of licensed OMV vaccines raises the question of whether such vaccines may provide the opportunity to control other outbreaks and epidemics. For instance, such a vaccine could control a localised outbreak of group B meningococci in Normandy, France. "Tailor-made" vaccines, focusing on the sub-capsular antigens may also be considered for use in sub-Saharan Africa for the prevention of the recurrent outbreaks by serogroups A and W135 meningococci. This assumption is based on the epidemiological observation that meningococcal outbreaks in Africa are clonal and are strikingly stable regarding their phenotypic characteristics.     

IgG antibody subclass responses determined by immunoblot in infants' sera following vaccination with a meningococcal recombinant hexavalent PorA OMV vaccine

The introduction of meningococcal serogroup C conjugate vaccines into the UK immunisation schedule has led to the decline of serogroup C disease in those vaccinated but there is no imminent vaccine solution for serogroup B disease. The PorA outer membrane protein (OMP) is a potential serogroup B vaccine candidate and an outer membrane vesicle (OMV) vaccine containing six different PorA OMPs (each representing a different serosubtype) has been evaluated in phase II trials with encouraging results. Little is known about the IgG subclass response to the various antigens contained within this vaccine. These responses are important due to the different half-lives and complement fixing abilities of these antibodies. In this study, immunoblotting was undertaken with infants' sera following either three or four doses of vaccine, and OMVs from six isogenic meningococcal strains differing only in their PorA serosubtype. Following either three or four doses of the vaccine, IgG(3) and IgG(1) subclass antibodies were induced to all six of the isogenic strains, although sera collected after four doses of vaccine showed stronger antibody levels. IgG(3) was found in more sera than IgG(1). For both sets of sera, the two isogenic strains expressing P1.5,2 and P1.5(c),10 induced stronger IgG subclass antibody responses than the other four meningococcal strains. The recombinant hexavalent PorA OMV vaccine stimulates both IgG(1) and IgG(3) subclass antibodies, the subclasses that are most effective in activating the complement system.     

Effect of sequence variation in meningococcal PorA outer membrane protein on the effectiveness of a hexavalent PorA outer membrane vesicle vaccine

Though meningococcal serogroup C conjugate vaccines have been introduced into the UK infant immunisation schedule, there is currently no vaccine solution for serogroup B disease. PorA outer membrane protein (OMP) is a potential serogroup B vaccine candidate. A hexavalent PorA outer membrane vesicle (OMV) vaccine has been evaluated in phase I and II trials with promising results. This vaccine contains six different PorA OMPs each representing a different serosubtype. However, considerable sequence variation occurs in the variable regions (VRs) encoding these serosubtypes. By using recombinant P1.5,10 PorA variants we have demonstrated that the killing of this particular serosubtype combination was due mainly to the induction of antibody to the VR2 (P1.10) epitope region, and that after three or four doses of vaccine there was a significant reduction in the killing of variants P1.10a (three doses, p<0.0001; four doses, p = 0.003) and P1.10f (three doses, p<0.0001; four doses, p = 0.002), as compared to responses to the P1.10 strain, when the P1.10 serosubtype was used as the immunogen. Since large numbers of serosubtype variants are known to exist, this finding may have implications for the use of PorA as a meningococcal serogroup B vaccine.     

Epidemiology of pathogenic Neisseria meningitidis serogroup B serosubtypes in Malta: implications for introducing PorA based vaccines

OBJECTIVE: To describe the epidemiology of the serosubtypes of Neisseria meningitidis serogroup B (MenB) in the most densely populated area in Europe and to review the MenB Porin A (PorA) based outer membrane vesicle (OMV) vaccines that could provide the broadest protection. STUDY DESIGN AND SETTING: Active surveillance of invasive meningococcal disease in a population of 400,000 inhabitants in Malta from 1999 to 2006. Serogroup B isolates were serosubtyped and analysed by age and year. The suitability of OMV vaccines was then assessed. RESULTS: Laboratory confirmation of invasive meningococcal disease was obtained in 48% (79/163) of notified cases. Serogroup B caused the majority of invasive meningococcal disease (76%, 60/79) with the greatest disease burden occurring in 0-14-year-old children (73%, 44/60). MenC caused 14% (11/79) of cases. The most prevalent MenB serotype:serosubtype combination was B:4:P1.19,15 which constituted 59% (34/58) of all phenotypeable MenB isolates. The PorA epitopes P1.15 and P1.19, detected in 74% (43/58) of isolates, were significantly more prevalent than serosubtypes with other PorA epitopes (chi(2): 7.18, P<0.01). CONCLUSION: An assessment of the usefulness of a MenB OMV vaccine in Malta requires further research. The wild-type OMV vaccine developed by the Finlay Institute (FI) in Cuba could potentially be used to control an outbreak with a MenB P1.19,15 clone. A multivalent OMV vaccine would however be needed for broader protection against the endemic heterogenous MenB strains. A serogroup B vaccine incorporating more conserved proteins than PorA would be more suitable for comprehensive control of meningococcal B disease.     

Effect of sequence variation in meningococcal PorA outer membrane protein on the effectiveness of a hexavalent PorA outer membrane vesicle vaccine in toddlers and school children

Though meningococcal conjugate vaccines are effective against serogroup C, there is currently no vaccine solution for serogroup B disease. PorA outer membrane protein (OMP) is a potential serogroup B vaccine candidate. A hexavalent PorA outer membrane vesicle (OMV) vaccine has been evaluated in phase I and II trials with promising results. However, considerable sequence variation occurs in the variable regions (VRs) encoding these serosubtypes. By using five wild type P1.19,15 variant strains we examined the serum bactericidal antibody (SBA) titres from sera collected from toddlers and school children pre- and post-vaccination. The numbers of subjects with SBA titres of <4, 4 and > or = 8 varied greatly between the different strains. This was also reflected when > or = 4-fold rises in SBA titres were examined. This finding in sera from toddlers and school children may have implications for PorA based vaccines.     

Immunogenicity and reactogenicity in UK infants of a novel meningococcal vesicle vaccine containing multiple class 1 (PorA) outer membrane proteins.

The development of effective vaccines against serogroup B meningococci is of great public health importance. We assessed a novel genetically engineered vaccine containing six meningococcal class 1 (PorA) outer membrane proteins representing 80% of prevalent strains in the UK. 103 infants were given the meningococcal vaccine at ages 2, 3 and 4 months with routine infant immunisations, with a fourth dose at 12-18 months. The vaccine was well tolerated. Three doses evoked good immune responses to two of six meningococcal strains expressing PorA proteins contained in the vaccine. Following a fourth dose, larger bactericidal responses to all six strains were observed, suggesting that the initial course had primed memory lymphocytes and revaccination stimulated a booster response. This hexavalent PorA meningococcal vaccine was safe and evoked encouraging immune responses in infants. Vaccines of this type warrant further development and evaluation.     

Immunogenicity of various presentation forms of PorA outer membrane protein of Neisseria meningitidis in mice.

In this study we compare different vaccine formulations containing meningococcal PorA outer membrane protein; purified PorA, outer membrane vesicles (OMV) and immune-stimulating complexes (iscom). Bactericidal antibodies could be generated by the OMV and iscom formulation but not with purified PorA using either A1PO4 or Quil-A as adjuvant. OMV and iscom formulations revealed similar immunogenicity when tested in a dose response manner, with respect to bactericidal as well as OMV-binding antibodies. The anti-OMV IgG subclass response induced by PorA in OMV formulation was found in all subclasses IgG1, IgG2a, IgG2b, IgG3. OMP-iscoms induced very high IgG1 anti-OMV antibodies but almost no IgG3 response. Also, OMP-iscoms appeared to be a potent inducer of antibodies directed against linear peptides corresponding to surface exposed loops of PorA. In addition, iscoms as well as purified PorA with Quil-A as adjuvant (but not with A1PO4) induced high levels of antibodies against purified PorA. In summary, in addition to the OMV formulation, only iscoms containing PorA are able to generate an anamnestic and bactericidal antibody response.     

Priorities for research on meningococcal disease and the impact of serogroup A vaccination in the African meningitis belt

For over 100 years, large epidemics of meningococcal meningitis have occurred every few years in areas of the African Sahel and sub-Sahel known as the African meningitis belt. Until recently, the main approach to the control of these epidemics has been reactive vaccination with a polysaccharide vaccine after an outbreak has reached a defined threshold and provision of easy access to effective treatment but this approach has not prevented the occurrence of new epidemics. Meningococcal conjugate vaccines, which can prevent meningococcal carriage and thus interrupt transmission, may be more effective than polysaccharide vaccines at preventing epidemics. Because the majority of African epidemics have been caused by serogroup A meningococci, a serogroup A polysaccharide/tetanus toxoid protein conjugate vaccine (PsA–TT) has recently been developed. Results from an initial evaluation of the impact of this vaccine on meningococcal disease and meningococcal carriage in Burkina Faso have been encouraging.     

A meningococcal NOMV-FHbp vaccine for Africa elicits broader serum bactericidal antibody responses against serogroup B and non-B strains than a licensed serogroup B vaccine

BackgroundMeningococcal epidemics in Sub-Sahara caused by serogroup A strains are controlled by a group A polysaccharide conjugate vaccine. Strains with serogroups C, W and X continue to cause epidemics. Protein antigens in licensed serogroup B vaccines are shared among serogroup B and non-B strains.PurposeCompare serum bactericidal antibody responses elicited by an investigational native outer membrane vesicle vaccine with over-expressed Factor H binding protein (NOMV-FHbp) and a licensed serogroup B vaccine (MenB-4C) against African serogroup A, B, C, W and X strains.MethodsHuman Factor H (FH) transgenic mice were immunized with NOMV-FHbp prepared from a mutant African meningococcal strain containing genetically attenuated endotoxin and a mutant sub-family B FHbp antigen with low FH binding, or with MenB-4C, which contains a recombinant sub-family B FHbp antigen that binds human FH, and three other antigens, NHba, NadA and PorA P1.4, capable of eliciting bactericidal antibody.ResultsThe NOMV-FHbp elicited serum bactericidal activity against 12 of 13 serogroup A, B, W or X strains from Africa, and four isogenic serogroup B mutants with sub-family B FHbp sequence variants. There was no activity against a serogroup B mutant with sub-family A FHbp, or two serogroup C isolates from a recent outbreak in Northern Nigeria, which were mismatched for both PorA and sub-family of the FHbp vaccine antigen. For MenB-4C, NHba was expressed by all 16 African isolates tested, FHbp sub-family B in 13, and NadA in five. However, MenB-4C elicited titers ≥1:10 against only one isolate, and against only two of four serogroup B mutant strains with sub-family B FHbp sequence variants.ConclusionsNOMV-FHbp has greater potential to confer serogroup-independent protection in Africa than the licensed MenB-4C vaccine. However, the NOMV-FHbp vaccine will require inclusion of sub-family A FHbp for coverage against recent serogroup C strains causing outbreaks in Northern Nigeria.     

Emergence of serogroup X meningococcal disease in Africa: Need for a vaccine

Neisseria meningitidis is responsible for the seasonal burden and recurrent epidemics of meningitis in an area of sub-Saharan Africa known as the meningitis belt. Historically, the majority of the cases in the meningitis belt are caused by serogroup A meningococci. Serogroup C meningococci were responsible for outbreaks in the meningitis belt in the 1980s, while serogroup W (formerly W-135) has emerged as a cause of epidemic meningitis since 2000. Serogroup X meningococci have previously been considered a rare cause of sporadic meningitis, but during 2006–2010, outbreaks of serogroup X meningitis occurred in Niger, Uganda, Kenya,Togo and Burkina Faso, the latter with at least 1300 cases of serogroup X meningitis among the 6732 reported annual cases. While serogroup X has not yet caused an epidemic wave of the scale of serogroup A in 1996–1997 or serogroup W in Burkina Faso during 2002, the existing reports suggest a similar seasonal hyperendemicity and capacity for localised epidemics. Serogroup X incidence appears to follow a pattern of highly localised clonal waves, and in affected districts, other meningococcal serogroups are usually absent from disease. Currently, no licensed vaccine is available against serogroup X meningococci. Following the introduction of a monovalent serogroup A conjugate vaccine (MenAfriVac^®) in the meningitis belt and the upcoming introduction of pneumococcal conjugate vaccines, vaccine-based prevention of serogroup X may become a public health need. The serogroup X polysaccharide capsule is the most likely target for vaccine development, but recent data also indicate a potential role for protein-based vaccines. A multivalent vaccine, preferably formulated as a conjugate vaccine and covering at least serogroups A, W, and X is needed, and the efforts for vaccine development should be intensified.     

Meningococcal surrogates of protection--serum bactericidal antibody activity

Despite the availability of anti-microbial agents effective against Neisseria meningitidis, meningococcal disease continues to be a major global health problem, particularly in the very young. Serogroup A meningococci cause large epidemics in sub-Saharan Africa, whilst serogroups B and C organisms are responsible for sporadic cases and localised outbreaks of disease world-wide. For measuring functional activity, the serum bactericidal antibody (SBA) assay is the most important method. It is mediated by antibody and complement resulting in lysis of the bacterial cells. To date the SBA has proved to be the best surrogate of protection for all serogroups. For serogroup C, an SBA titre of either >/ or =4 or > or =8 has being utilised for putatively indicating protection when using either human or baby rabbit complement, respectively. For serogroup B, the proportions of vaccines with > or =4-fold rises in SBA pre- to post-vaccination or SBA titres > or =4 have been correlated with clinical efficacy in trials of outer membrane vesicle (OMV) vaccines in Cuba, Brazil and Norway. SBA activity as a correlate of protection for evaluating the immune response to meningococcal vaccines is described in this review.     

Avidity maturation following vaccination with a meningococcal recombinant hexavalent PorA OMV vaccine in UK infants

To date, there are no data assessing the utility of avidity indices as a surrogate marker for the induction of immunological memory following meningococcal serogroup B outer membrane vesicle (OMV) vaccination. We studied infants who had been immunized with three doses of a recombinant hexavalent PorA OMV vaccine at ages 2-4 months, together with a fourth dose at age 12-18 months. A control group had received a single dose of the same vaccine at age 12-18 months. As previously reported, serum bactericidal antibody (SBA) titres increased after each of the first three doses, with a significant increase observed from 6 months post third dose to 1 month post fourth dose. The geometric mean avidity indices (GMAI), against strain H44/76 OMVs, increased from 1 month post first dose to 1 month post third dose. Significant increases in GMAI were observed at 6 months post third dose and again following the fourth dose. At 32-42 months of age, though the SBA titres had returned to post first dose levels, the GMAI remained elevated. No increase in avidity was observed in the control group. Antibody avidity indices are useful laboratory markers for the priming of immunological memory following vaccination with meningococcal serogroup B OMV vaccines.     

Neisseria meningitidis serogroup C polysaccharide and serogroup B outer membrane vesicle conjugate as a bivalent meningococcus vaccine candidate.

Neisseria meningitidis serogroup C polysaccharide (PS C) was conjugated to serogroup B outer membrane vesicles (OMV) in order to test the possibility of obtaining a bivalent group B and C meningococcus vaccine. The conjugate and controls were injected intraperitoneally into groups of ten mice with boosters on days 14 and 28 after the primary immunization. The following groups were used as control: (i) PS C; (ii) PS C plus OMV; (iii) OMV; and (iv) saline. The serum collected on days 0, 14, 28 and 42 were tested by enzyme-linked immunosorbent assay (ELISA) for PS C and OMV, and by complement mediated bactericidal assay against serogroups B and C. ELISA for PS C as well as bactericidal titres against serogroup C meningococci of the conjugated vaccine increased eight-fold (ELISA) and 32 fold (bactericidal) after 42 days in comparison with the PS C control group. ELISA for OMV and bactericidal titre against serogroup B meningococci of the conjugate showed no significant difference in comparison with the OMV containing controls. Furthermore, Western Blot assay of the conjugate immune serum did not bind OMV class four protein which is related to the complement dependent antibody suppressor. The results indicate that the PS C-OMV conjugate could be a candidate for a bivalent vaccine toward serogroups B and C meningococci.     

Improved OMV vaccine against Neisseria meningitidis using genetically engineered strains and a detergent-free purification process

The use of detergent-extracted outer membrane vesicles (OMVs) is an established approach for development of a multivalent PorA vaccine against N. meningitidis serogroup B. Selective removal of lipopolysaccharide (LPS) decreases toxicity, but promotes aggregation and narrows the immune response. Detergent-free OMV vaccines retain all LPS, which preserves the native vesicle structure, but result in high toxicity and lower yield. The present study assessed the effects of gene mutations that attenuated LPS toxicity (lpxL1) or improved OMV yield (rmpM) in combination with the available OMV purification processes. The results substantiate that OMVs from a strain with both mutations, produced with a detergent-free process provide better vaccine characteristics than the traditional detergent-based approach. With comparable toxicity and yield, no aggregation and cross-protection against other PorA subtypes, these OMV vaccines are potentially safe and effective for parenteral use in humans.     

Evaluation of serogroup A meningococcal vaccines in Africa: a demonstration project

Endemic and epidemic meningococcal disease constitutes a major public-health problem in African countries of the 'meningitis belt' where incidence rates of the disease are many-fold higher (up to 25 cases per 100,000 population) than those in industrialized countries, and epidemics of meningococcal disease occur with rates as high as 1,000 cases per 100,000 people. Using the precedent established during the licensing of conjugate vaccines against Haemophilus influenzae type b and serogroup C meningococci and components of currently-licensed meningococcal polysaccharide vaccines, new meningococcal conjugate vaccines will likely be licensed using immunological endpoints as surrogates for clinical protection. Post-licensure evaluation of vaccine effectiveness will, therefore, be of increased importance. One vaccine being developed is the serogroup A meningococcal (Men A) conjugate vaccine produced by the Meningitis Vaccine Project (MVP), a partnership between the World Health Organization and the Program for Applied Technology in Health. This vaccine will likely be the first meningococcal conjugate vaccine introduced on a large scale in Africa. This paper summarizes the general steps required for vaccine development, reviews the use of immunogenicity criteria as a licensing strategy for new meningococcal vaccines, and discusses plans for evaluating the impact of a meningococcal A conjugate vaccine in Africa. Impact of this vaccine will be measured during a vaccine-demonstration project that will primarily measure the effectiveness of vaccine. Other studies will include evaluations of safety, vaccine coverage, impact on carriage and herd immunity, and prevention-effectiveness studies.     

Immunogenicity studies with a genetically engineered hexavalent PorA and a wild-type meningococcal group B outer membrane vesicle vaccine in infant cynomolgus monkeys

The immunogenicity of two meningococcal outer membrane vesicle (OMV) vaccines, namely the Norwegian wild-type OMV vaccine and the Dutch hexavalent PorA OMV vaccine, were examined in infant cynomolgus monkeys. For the first time, a wild-type- and a recombinant OMV vaccine were compared. Furthermore, the induction of memory and the persistence of circulating antibodies were measured. The Norwegian vaccine contained all four classes of major outer membrane proteins (OMP) and wild-type L3/L8 lipopolysaccharide (LPS). The Dutch vaccine consisted for 90% of class 1 OMPs, had low expression of class 4 and 5 OMP, and GalE LPS. Three infant monkeys were immunised with a human dose at the age of 1.5, 2.5 and 4.5 months. Two monkeys of each group received a fourth dose at the age of 11 months. In ELISA, both OMV vaccines were immunogenic and induced booster responses, particularly after the fourth immunisation. The Norwegian vaccine mostly induced sero-subtype P1.7,16 specific serum bactericidal antibodies (SBA), although some other SBA were induced as well. The antibody responses against P1.7,16, induced by the Norwegian vaccine, were generally higher than for the Dutch vaccine. However, the Dutch vaccine induced PorA specific SBA against all six sero-subtypes included in the vaccine showing differences in the magnitude of SBA responses to the various PorAs.     

Meningococcal vaccine development--from glycoconjugates against MenACWY to proteins against MenB--potential for broad protection against meningococcal disease

Novartis Vaccines has a long-standing research and development interest in the prevention of invasive meningococcal disease. From the initial licensure of the monovalent meningococcal C glycoconjugate vaccine, Menjugate(?), in response to the emergence of a virulent serogroup C ST-11 strain in the United Kingdom to the more recent development and licensure of a quadrivalent meningococcal ACWY glycoconjugate vaccine, Menveo(?), Novartis has a continuing commitment to the development of more effective tools for the control of meningococcal disease. Menveo is now licensed for use in adolescents and adults in over 50 countries and results from phase III studies have shown the vaccine to be well-tolerated and highly immunogenic in infants with vaccination beginning from 2 months of age. The 'holy grail' of meningococcal disease control is a broadly protective vaccine against serogroup B (MenB), preferably a vaccine that protects all age groups including infants. As the serogroup B capsule is poorly immunogenic, efforts over the past 40 years have focused on identifying conserved proteins expressed on the bacterial surface that elicit bactericidal antibodies. Novartis has approached this problem utilizing genomic tools to identify proteins meeting these criteria in a process now known as 'reverse vaccinology'[1]. This process has resulted in a novel multicomponent MenB vaccine (4CMenB) that consists of four major immunogenic components (three subcapsular MenB protein antigens plus outer membrane vesicles (OMVs) which themselves provide multiple subcapsular antigens, the immunodominant one being PorA). These all induce bactericidal antibodies against the antigens that are important in determining the survival, function, and virulence of the meningococci. Phase II studies of 4CMenB have been completed and have demonstrated that the vaccine is highly immunogenic against reference meningococcal strains selected to support licensure. Post-vaccination sera from clinical studies have also been tested against a diverse panel of serogroup B strains to support the development of the Meningococcal Antigen Typing System (MATS), a tool used to predict vaccine strain coverage [2] This overview is intended to give a broad summary of the key clinical data derived from the Menveo and 4CMenB clinical development programs.     

Reverse vaccinology--in search of a genome-derived meningococcal vaccine

Although significant advances have been made toward the control of bacterial meningitis in children with the development of capsular polysaccharide protein conjugate vaccines, this approach has proven problematic for the serogroup B meningococcus. Non-capsular vaccines based upon outer membrane vesicles of Neisseria meningitidis have been useful in control of clonal serogroup B outbreaks, although due to variability of PorA, these vaccines may be less useful in control of endemic disease. Genome-based vaccine discovery was evaluated in an attempt to produce a candidate capable of conferring a broadly protective vaccine against a diversity of meningococcal B strains.     

Immunogenicity and safety of monovalent p1.7(h),4 meningococcal outer membrane vesicle vaccine in toddlers: comparison of two vaccination schedules and two vaccine formulations

The safety and immunogenicity of two PorA-based meningococcal outer membrane vesicle (OMV) vaccines against the P1.4 serosubtype adsorbed with AlPO(4) or Al(OH)(3) were studied in 134 toddlers. Vaccinations were given three times with an interval of 3-6 weeks or twice with an interval of 6-10 weeks. A vaccination was repeated after 20-40 weeks. Pre- and post-immunization sera were tested for bactericidal activity against an isogenic strain expressing P1.7(h), 4 PorA. Both meningococcal OMV vaccines were well tolerated. The percentage of children with a fourfold increase in bactericidal activity was 96% (AlPO(4)-adjuvated vaccine/2+1 schedule), 100% (AlPO(4)-adjuvated vaccine/3+1 schedule), 93% (Al(OH)(3)-adjuvated vaccine/2+1 schedule) and 97% (Al(OH)(3)-adjuvated vaccine/3+1 schedule). Adsorption with AlPO(4) makes the OMV vaccine more immunogenic than adsorption with Al(OH)(3). Bactericidal activity was highest after the 3+1 schedule, although the response shortly after the primary series was higher in the two-dose priming group.