Investigation of different group A immunoassays following one dose of meningococcal group A conjugate vaccine or A/C polysaccharide vaccine in adults

Evaluating Functional Immunity Following Encapsulated Bacterial Infection and Vaccination

Encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae type b and Neisseria meningitidis cause significant morbidity and mortality in young children despite the availability of vaccines. Highly specific antibodies are the primary mechanism of protection against invasive disease. Robust and standardised assays that measure functional antibodies are also necessary for vaccine evaluation and allow for the accurate comparison of data between clinical studies. This mini review describes the current state of functional antibody assays and their importance in measuring protective immunity.

Bactericidal antibodies against hypervirulent Neisseria meningitidis C field strains following MenC-CRM or MenACWY-CRM priming and MenACWY-CRM booster in children

An increase in invasive meningococcal disease (IMD) incidence was observed in Tuscany in 2015/2016, mainly due to hypervirulent clonal complex (cc) 11 strains. In a post-hoc analysis, we assessed bactericidal activity of antibodies in sera from children primed with MenACWY-CRM or MenC-CRM conjugate vaccines and receiving a MenACWY-CRM booster dose against 5 meningococcal C (MenC) strains isolated from IMD cases. Sera collected from 90 infants/toddlers who participated in a phase III, open-label study (NCT00667602) and its extension (NCT01345721) were tested by serum bactericidal activity assay with human complement (hSBA). Children were primed with either MenACWY-CRM at 6–8 and 12 months of age (group 2_MenACWY; N = 30), MenACWY-CRM (group 1_MenACWY; N = 30), or MenC-CRM at 12 months of age (group 1_MenC; N = 30); all received MenACWY-CRM booster dose at 22–45 months of age. Four tested strains (FI001–FI004) were C:P1.5–1,10-8:F3-6:ST-11 (cc11) and 1 (FI005) was C:P1.7–4,14-6:F3-9:ST-1031 (cc334). Overall, immune responses tended to be higher against Fl002–FI004 than Fl001 and Fl005. Geometric mean titers were high in group 2_MenACWY (range: 94.8 [FI005]–588.1 [FI004]) and very high post-boosting with MenACWY-CRM in all groups (176.9 [FI005]–3911.0 [FI004]). Seroresponse rates tended to be higher in group 1_MenC (33.3% [FI005]–93.3% [FI004]) than in group 1_MenACWY (16.7% [FI005]–73.3% [FI004]). Irrespective of strains tested or the identity/number of priming doses, ≥96.7% of children had hSBA titers ≥1:8 post-MenACWY-CRM booster dose. MenACWY-CRM and MenC-CRM elicited bactericidal antibodies and immunological memory against hypervirulent cc11 and cc334 MenC strains responsible for IMD outbreaks.

Immunogenicity and safety of a quadrivalent meningococcal tetanus toxoid-conjugate vaccine (MenACYW-TT) in adults 56 years of age and older: a Phase II randomized study

MenACYW-TT is an investigational quadrivalent meningococcal conjugate vaccine intended for the prevention of invasive meningococcal disease (IMD) caused by serogroups A, C, W, and Y in individuals aged 6 weeks and above. This Phase II, randomized, open-label, multicenter, exploratory study assessed the safety and immunogenicity of MenACYW-TT compared with a quadrivalent meningococcal polysaccharide vaccine (MPSV4) in 301 healthy adults aged ≥56 y in the US (NCT01732627). Participants were randomized 2:1 to receive MenACYW-TT or MPSV4. Serum bactericidal assays using human (hSBA) or baby rabbit (rSBA) complement were used to measure functional antibodies against meningococcal serogroups A, C, W, and Y at baseline and 30 d post-vaccination. Safety data were collected up to 30 d post-vaccination. Proportions of study participants with hSBA titers ≥1:8 against serogroups A, C, W, and Y were increased at Day 30 compared with baseline in both vaccine groups. The proportions of participants with hSBA titers ≥1:8 after MenACYW-TT vaccination were comparable to those after MPSV4 vaccination for serogroups A and C (A: 93.8% vs. 85.1%; C: 74.9% vs. 62.8%) and distinctly higher than after MPSV4 for serogroups W and Y (W: 79.5% vs. 60.6%; Y: 80.5% vs. 59.6%). Proportions of participants with rSBA titers ≥1:8 were comparable between vaccine groups for all four serogroups. The reactogenicity profiles of both vaccines were similar. Most unsolicited adverse events (AEs) were of Grade 1 or Grade 2 intensity, and no serious AEs were reported. The MenACYW-TT conjugate vaccine was well tolerated and immunogenic in adults aged ≥56 y.

Immunogenicity and safety of a quadrivalent meningococcal tetanus toxoid-conjugate vaccine (MenACYW-TT) in healthy toddlers: a Phase II randomized study

NEISSERIA MENINGITIDIS

can lead to invasive meningococcal disease to which young children are particularly vulnerable. We assessed the immunogenicity and safety of Sanofi Pasteur's investigational quadrivalent (serogroups A, C, Y, and W) meningococcal tetanus-toxoid conjugate vaccine, MenACYW-TT, as a single dose, in healthy meningococcal vaccine-naïve toddlers versus a licensed conjugate vaccine MCV4-TT (NCT03205358). In this Phase II study conducted in Finland, 188 toddlers aged 12-24 months were randomized 1:1 to MenACYW-TT or MCV4-TT. Serum bactericidal antibody assays using human complement (hSBA) and baby rabbit complement (rSBA) measured antibodies against each serogroup before and 30 days after vaccination. Participants were monitored for immediate adverse events (AEs) and post-vaccination AEs for 30 days. All analyses were descriptive. All 188 participants completed the study. The Day 30 hSBA seroresponses (hSBA titer <8 at baseline and post-vaccination titer ≥8, or ≥8 at baseline and ≥4-fold increase post-vaccination) were comparable between participants receiving MenACYW-TT (96.7-100%), and MCV4-TT (86.0-100.0%) for each serogroup. Most unsolicited AEs were of Grade 1 or Grade 2 intensity. There were no immediate hypersensitivity reactions, and no AEs or serious AEs leading to discontinuation from the study. In this exploratory study, MenACYW-TT vaccine was well tolerated and immunogenic. If confirmed in Phase III, a single dose of the MenACYW-TT vaccine may show promise as an alternative vaccine option for toddlers receiving meningococcal vaccination for the first time.

Association of serum bactericidal antibody and opsonic antibody levels after Neisseria meningitidis serogroup C conjugate vaccine in Brazilian children and adolescents infected or not infected with HIV

Neisseria meningitidis serogroup C (MenC) is the main causative agent of meningitis in Brazil. HIV infection affects the quality of the immune system. HIV⁺ children have an increased risk of infection to encapsulated bacteria such as N. meningitidis. We evaluated the opsonic antibody (OPA) levels and its correlation with serum bactericidal antibody (SBA) levels induced by one and two doses of a MenC conjugate vaccine in children and adolescents HIV⁺ and HIV-exposed but uninfected children (HEU) group. Overall the data show the importance of two doses of vaccine for HIV⁺ individuals. About 79% and 58% of HIV⁺ patients showed SBA and OPA positive response after two doses of vaccine, respectively. For HEU group, 62% and 41% of patients showed SBA and OPA positive response after one dose of vaccine, respectively. A positive and significant association between SBA and OPA levels was seen after two doses of vaccine in HIV⁺ patients.

Persistence of the immune response after MenACWY-CRM vaccination and response to a booster dose, in adolescents, children and infants

Persistence of bactericidal antibodies following vaccination is extremely important for protection against invasive meningococcal disease, given the epidemiology and rapid progression of meningococcal infection. We present an analysis of antibody persistence and booster response to MenACWY-CRM, in adolescents, children and infants, from 7 clinical studies. Immunogenicity was assessed using the serum bactericidal assay with both human and rabbit complement. Post-vaccination hSBA titers were high, with an age- and serogroup-specific decline in titers up to 1 y and stable levels up to 5 y The waning of hSBA titers over time was more pronounced among infants and toddlers and the greatest for serogroup A. However, rSBA titers against serogroup A were consistently higher and showed little decline over time, suggesting that protection against this serogroup may be sustained. A single booster dose of MenACWY-CRM administered at 3 to 5 y induced a robust immune response in all age groups.

Comparative Assessment of a Single Dose and a 2-dose Vaccination Series of a Quadrivalent Meningococcal CRM-conjugate Vaccine (MenACWY-CRM) in Children 2-10 Years of Age

BACKGROUND

We compared the immunogenicity, safety and 1-year antibody persistence of a single-dose and a 2-dose series of a licensed meningococcal ACWY-CRM conjugate vaccine (MenACWY-CRM) in 2- to 10-year-old children.

METHODS

In this phase III, multicenter, observer-blind study, children aged 2-5 years (n = 359) and 6-10 years (n = 356) were randomized 1:1 to receive 2 doses of MenACWY-CRM (ACWY2) or 1 dose of placebo followed by 1 dose of MenACWY-CRM (ACWY1), 2 months apart. Immunogenicity was measured using serum bactericidal activity with human complement (hSBA). Primary outcomes were to assess the immunologic noninferiority and superiority of ACWY2 versus ACWY1.

RESULTS

One-month after the second dose, the hSBA seroresponse in ACWY2 was noninferior to ACWY1 for all 4 serogroups, in both age cohorts, and was superior for serogroups C and Y in the 2- to 5-year-old age cohort and for serogroup Y in the 6- to 10-year-old age cohort. Overall, 90%-99% of subjects in ACWY2 and 65%-96% in ACWY1 had hSBA titers ≥ 8; geometric mean titers were 1.8- to 6.4-fold higher in ACWY2 than ACWY1 across serogroups. At 1 year postvaccination, geometric mean titers declined, and the differences between ACWY2 and ACWY1 remained significant for serogroups A and C in the 2- to 5-year-old age cohort and for serogroups C and Y in the 6- to 10-year-old age cohort. The safety profile of MenACWY-CRM was similar in both groups.

CONCLUSIONS

The single dose and 2-dose MenACWY-CRM series were immunogenic and well tolerated. Although antibody responses were greater after 2 doses, especially in the 2- to 5-year-old age cohort, this difference was less pronounced at 1 year postvaccination.

Antibody persistence 5 years after vaccination at 2 to 10 years of age with Quadrivalent MenACWY-CRM conjugate vaccine, and responses to a booster vaccination

BACKGROUND

In a multi-center extension study, children 2-10 years of age, initially vaccinated with one or two doses (2-5 year-olds) or one dose (6-10 year-olds) of quadrivalent meningococcal CRM197-conjugate vaccine (MenACWY-CRM), were assessed five years later for antibody persistence and booster response using serum bactericidal assay with human complement (hSBA).

METHODS

Children 7-10 and 11-15 years of age, who received MenACWY-CRM in the original study, and age-matched vaccine-naïve children, were enrolled in this extension study. After an initial blood draw, children received one dose of MenACWY-CRM as booster or primary dose, with a second blood draw 28 days later.

RESULTS

hSBA titers decreased five years after primary vaccination, but were higher than in non-vaccinated controls against serogroups C, W and Y, with substantial proportions having titers ≥8: 7-22% for A, 32-57% for C, 74-83% for W, and 48-54% for Y. Previously-vaccinated children demonstrated booster responses to revaccination against all four serogroups. Responses to primary vaccination in vaccine-naïve controls were lower and similar to primary responses observed in the original study. All vaccinations were generally well tolerated, with no safety concern raised.

CONCLUSIONS

Approximately half the children vaccinated as 2-10 year-olds maintained protective antibodies against serogroups C, W and Y five years later, but fewer did against serogroup A. Declining titers five years after vaccination and robust booster responses suggest that five years may be an appropriate interval to revaccinate children, subject to epidemiology and delivery considerations.

Antibody persistence after primary and booster doses of a quadrivalent meningococcal conjugate vaccine in adolescents

BACKGROUND

The aim of this study was to evaluate antibody persistence 5 years after primary vaccination with the quadrivalent meningococcal conjugate vaccines MenACWY-CRM or MenACWY-D and 2 years after a booster dose of MenACWY-CRM, in the context of a phase 3 study.

METHODS

Subjects (aged 19.2 ± 2.3 years) were assigned to 5 groups according to whether they had previously received primary vaccination (at 14.2 ± 2.2 years) with MenACWY-CRM (N = 131) or MenACWY-D (N = 76), a booster dose of MenACWY-CRM 3 years after primary vaccination with MenACWY-CRM (N = 44) or MenACWY-D (N = 31) or no vaccination (N = 107). The immunogenicity measures were percentages of subjects with serum bactericidal activity (hSBA) ≥ 1:8 for serogroups A, C, W and Y and hSBA geometric mean titers. Comparisons with age-matched, vaccine-naive subjects were performed.

RESULTS

A majority of subjects vaccinated 5 years previously maintained hSBA ≥ 1:8 against serogroups C, W and Y in the MenACWY-CRM (59%-82%) and MenACWY-D groups (54%-73%); this was lower for serogroup A in both groups. There was a decline in antibody titers after primary vaccination, especially in the first 2 years postprimary vaccination, with steady concentrations during the next 3 years. Two years after MenACWY-CRM booster vaccination the percentages of subjects with hSBA ≥ 1:8 ranged from 77% to 100% across serogroups and geometric mean titers were 2.5- to 8-fold higher than prebooster values across serogroups.

CONCLUSIONS

Booster vaccination with MenACWY-CRM elicited a robust immune response during the 2-year follow-up period, irrespective of previous vaccination.

Evaluation and validation of a serum bactericidal antibody assay for Haemophilus influenzae type b and the threshold of protection

Prior to routine immunisation, Haemophilus influenzae serotype b (Hib) was a major cause of serious bacterial infections, particularly in young children. In the United Kingdom, introduction of the Hib conjugate vaccine into the national childhood immunisation schedule has led to a sustained decline in invasive Hib disease across all age-groups. Evaluation of the immune response to Hib conjugate vaccines involves measurement of serum IgG antibodies against the capsular polyribosyl-ribitol-phosphate (PRP) polysaccharide by enzyme-linked immunosorbent assay (ELISA), with accepted short-term and long-term protective thresholds of ≥0.15μg/mL and ≥1.0μg/mL, respectively. These levels were derived by passive immunisation or immunisation with pure polysaccharide, and their relevance for protection following immunisation with conjugate vaccines remains unclear. This study aimed to modify and optimise a serum bactericidal antibody (SBA) assay to evaluate the functional activity of Hib antibodies generated following Hib conjugate vaccination. Validation of the Hib SBA assay was deemed acceptable for all assay parameters tested. A strong correlation between anti-PRP IgG concentrations and SBA titres was observed in vaccinated adults (r=0.81), as well as infants after primary immunisation at 2, 3, and 4 months (r=0.635) and after the 12-month booster (r=0.746). The assay identified some children with high anti-PRP IgG but low SBA activity and vice versa. The predictive protective SBA titre corresponding to a post-booster anti-PRP IgG of 1.0μg/mL was 8. Thus, the optimised Hib SBA assay was specific and reproducible and correlated with anti-PRP IgG. Such assays may have a role in evaluating immune responses to conjugate vaccines in addition to measuring capsular antibodies.

Avidity of IgG antibodies against meningococcal serogroup a polysaccharide and correlations with bactericidal activity in sera from meningitis patients and controls from Ethiopia

Meningococcal meningitis is a significant global health challenge, especially for sub-Saharan area: the African meningitis belt. Neisseria meningitidis of serogroup A (MenA) is responsible for the large number of epidemics that have been recorded in these countries. To determine the level of antibodies against meningococcal A polysaccharide (APS) that correlates with protection against MenA disease in the African meningitis belt, it may be important to consider antibody avidity along with quantity. In this study, two ELISA methods using the chaotropic agent ammonium thiocyanate were compared and employed to measure avidity indexes (AI) of IgG antibodies against APS in controls and in acute and convalescent sera from Ethiopian meningococcal patients. High statistical correlations between the AIs determined by the two methods were observed. The geometric mean AI (GMAI) increased with time from acute to convalescent sera indicating affinity maturation. GMAI was significantly higher in convalescent sera from the MenA patients and in sera from the controls than in acute sera from patients with meningococcal disease. A significant correlation between serum bactericidal activity titres (SBA) and concentration of IgG antibodies against APS was observed; however, our results did not indicate that determination of antibody avidities by the thiocyanate elution method gave a better correlation with SBA than anti-APS IgG concentrations determined by the standard ELISA method.

Antibody persistence and booster response of a quadrivalent meningococcal conjugate vaccine in adolescents

OBJECTIVE

To evaluate the tolerability and immunogenicity of a booster dose of the quadrivalent meningococcal conjugate vaccine MenACWY-CRM (Menveo, Novartis Vaccines and Diagnostics, Siena, Italy) administered 3 years after primary vaccination of adolescents enrolled in a phase 3 study with either MenACWY-CRM or MenACWY-D (Menactra, Sanofi Pasteur, Swiftwater, Pennsylvania).

STUDY DESIGN

A total of 730 healthy adolescents participated, including 622 initial study participants who received primary vaccination with MenACWY-CRM (n = 367) or MenACWY-D (n = 255) 3 years previously and 108 age-matched vaccine-naïve controls. A subset of MenACWY-CRM (n = 83) and MenACWY-D (n = 77) recipients were administered a MenACWY-CRM booster dose 3 years postprimary vaccination. Immunogenicity prior to and after the booster dose of MenACWY-CRM was measured by serum bactericidal assay with human complement (hSBA). Local and systemic reactions and adverse events were monitored in subjects receiving the booster dose.

RESULTS

At 3 years postprimary vaccination, 64%, 82%, and 65% of subjects initially vaccinated with MenACWY-CRM (n = 367) showed hSBA titers ≥8 against serogroups C, W-135, and Y, respectively; this was lower for serogroup A (28%). Significantly more MenACWY-CRM recipients had hSBA titers ≥8 for serogroups W-135 and Y than MenACWY-D recipients (n = 255). A MenACWY-CRM booster dose resulted in 99%-100% of subjects demonstrating hSBA titers ≥8 against all serogroups, irrespective of primary vaccination (MenACWY-CRM, n = 83; MenACWY-D, n = 77). The booster dose was well tolerated without significant adverse events.

CONCLUSIONS

MenACWY-CRM can be used to boost adolescents who have received a primary vaccination with either MenACWY-CRM or MenACWY-D.

Development and use of a serum bactericidal assay using pooled human complement to assess responses to a meningococcal group A conjugate vaccine in African toddlers

A meningococcal group A polysaccharide (PS) conjugate vaccine (PsA-TT) has been developed for African countries affected by epidemic meningitis caused by Neisseria meningitidis. Complement-mediated serum bactericidal antibody (SBA) assays are used to assess protective immune responses to meningococcal vaccination. Human complement (hC') was used in early studies demonstrating antibody-mediated protection against disease, but it is difficult to obtain and standardize. We developed and evaluated a method for sourcing hC' and then used the SBA assay with hC' (hSBA) to measure bactericidal responses to PsA-TT vaccination in 12- to 23-month-old African children. Sera with active complement from 100 unvaccinated blood donors were tested for intrinsic bactericidal activity, SBA titer using rabbit complement (rSBA), and anti-group A PS antibody concentration. Performance criteria and pooling strategies were examined and then verified by comparisons of three independently prepared hC' lots in two laboratories. hSBA titers of clinical trial sera were then determined using this complement sourcing method. Two different functional antibody tests were necessary for screening hC'. hSBA titers determined using three independent lots of pooled hC' were within expected assay variation among lots and between laboratories. In African toddlers, PsA-TT elicited higher hSBA titers than meningococcal polysaccharide or Hib vaccines. PsA-TT immunization or PS challenge of PsA-TT-primed subjects resulted in vigorous hSBA memory responses, and titers persisted in boosted groups for over a year. Quantifying SBA using pooled hC' is feasible and showed that PsA-TT was highly immunogenic in African toddlers.

Immunogenicity and safety of a CRM-conjugated meningococcal ACWY vaccine administered concomitantly with routine vaccines starting at 2 months of age

BACKGROUND

Infants are at the highest risk for meningococcal disease and a broadly protective and safe vaccine is an unmet need in this youngest population. We evaluated the immunogenicity and safety of a 4-dose infant/toddler regimen of MenACWY-CRM given at 2, 4, 6, and 12 months of age concomitantly with pentavalent diphtheria-tetanus-acellular pertussis-Hemophilus influenzae type b-inactivated poliovirus-combination vaccine (DTaP-IPV/Hib), hepatitis B vaccine (HBV), 7- or 13-valent conjugate pneumococcal vaccine (PCV), and measles, mumps, and rubella vaccine (MMR).

RESULTS

Four doses of MenACWY-CRM induced hSBA titers ≥8 in 89%, 95%, 97%, and 96% of participants against serogroups A, C, W-135, and Y, respectively. hSBA titers ≥8 were present in 76-98% of participants after the first 3 doses. A categorical linear analysis incorporating vaccine group and study center showed responses to routine vaccines administered with MenACWY-CRM were non-inferior to routine vaccines alone, except for seroresponse to the pertussis antigen fimbriae. The reactogenicity profile was not affected when MenACWY-CRM was administered concomitantly with routine vaccines.

CONCLUSION

MenACWY-CRM administered with routine concomitant vaccinations in young infants was well tolerated and induced highly immunogenic responses against each of the serogroups without significant interference with the immune responses to routine infant vaccinations.

METHODS

Healthy 2 month old infants were randomized to receive MenACWY-CRM with routine vaccines (n = 258) or routine vaccines alone (n = 271). Immunogenicity was assessed by serum bactericidal assay using human complement (hSBA). Medically attended adverse events (AEs), serious AEs (SAEs) and AEs leading to study withdrawal were collected throughout the study period.

Persistence of antibodies in laboratory staff immunized with quadrivalent meningococcal polysaccharide vaccine

BACKGROUND

Occupational exposure to live meningococci can potentially cause invasive meningococcal disease in laboratory staff. While, until recently, immunization with quadrivalent polysaccharide vaccine represented one cornerstone of protection, data on long-term persistence of antibodies in adults remain scarce.

METHODS

We analyzed the relationship of antibody levels and time following quadrivalent polysaccharide vaccination (Mencevax® ACWY, GlaxoSmithKline) in a cross-sectional sample of 20 laboratory workers vaccinated at ages between 16.4 to 40.7 years from Germany. Sera were obtained 0.4 to 158.5 (median 35.3) months after vaccination. At the time of sampling, laboratory workers had been regularly exposed to meningococci for periods between 3.2 to 163.8 (median 41.2) months. Serum bactericidal assay (SBA) with rabbit complement and a microsphere-based flow analysis method were used to determine bactericidal titers and concentrations of IgG, respectively, against serogroups A, C, W135, and Y. Decay of antibodies was modeled using linear regression. Protective levels were defined as SBA titers ≥ 8.

RESULTS

Half-lives of SBA titers against serogroups A, C, W135, and Y were estimated at 27.4, 21.9, 18.8, and 28.0 months, respectively. Average durations of protection were estimated at 183.9, 182.0, 114.6, and 216.4 months, respectively. Inter-individual variation was high; using lower margins of 95% prediction intervals, minimal durations of protection against serogroups A, C, W135 and Y were estimated at 33.5, 24.6, 0.0, and 55.1 months, respectively. The proportion of staff with protective SBA titers against W135 (65.0%) was significantly lower than proportions protected against A (95.0%), C (94.7%), and Y (95.0%). Consistently, geometric mean titer (97.0) and geometric mean concentration of IgG (2.1 μg/ml) was lowest against serogroup W135. SBA titers in a subset of individuals with incomplete protection rose to ≥ 128 (≥ 8 fold) after reimmunization with a quadrivalent glycoconjugate vaccine.

CONCLUSIONS

The average duration of protection following immunization with a quadrivalent polysaccharide vaccine in adults was ≥ 115 months regardless of serogroup. A substantial proportion (approximately 23% according to our decay model) of adult vaccinees may not retain protection against serogroup W135 for five years, the time suggested for reimmunization.

Development of a large scale human complement source for use in bacterial immunoassays

The serum bactericidal assay is the correlate of protection for meningococcal disease but the use and comparison of functional immunological assays for the assessment of meningococcal vaccines is complicated by the sourcing of human complement. This is due to high levels of immunity in the population acquired through natural meningococcal carriage and means that many individuals must be screened to find donors with suitably low bactericidal titres against the target strain. The use of different donors for each meningococcal strain means that comparisons of assay responses between strains and between laboratories is difficult. We have developed a method for IgG-depletion of 300 ml batches of pooled human lepirudin-derived plasma using Protein G sepharose affinity chromatography that retains complement activity. However, IgG-depletion also removed C1q. This was also eluted from the affinity matrix, concentrated and added to the complement source. The final complement source retained mean alternative pathway activity of 96.8% and total haemolytic activity of 84.2% in four batches. Complement components C3, C5, properdin and factor H were retained following the process and the IgG-depleted complement was shown to be suitable for use in antibody-mediated complement deposition and serum bactericidal activity assays against serogroup B meningococci. The generation of large IgG-depleted batches of pooled human plasma allows for the comparison of immunological responses to diverse meningococcal strain panels in large clinical trials.

Safety and immunogenicity of a meningococcal quadrivalent conjugate vaccine in five- to eight-year-old Saudi Arabian children previously vaccinated with two doses of a meningococcal quadrivalent polysaccharide vaccine

Saudi Arabian children respond poorly to 2 doses of meningococcal quadrivalent polysaccharide vaccine (MPSV4) when given before 2 years of age. This study examined whether such children were able to respond to 1 dose of quadrivalent meningococcal diphtheria toxoid conjugate vaccine (MCV4) when they were older. Saudi Arabian children 5 to 8 years of age who had previously been vaccinated with 2 doses of MPSV4 when they were under 2 years of age (termed the prior-MPSV4 group) were enrolled in a controlled, open-label, multicenter study. In the prior-MPSV4 group, children (n = 153) received 1 dose of MCV4, as did a group of age-matched meningococcal vaccine-naïve children (n = 85). Blood samples collected prevaccination and 28 days postvaccination were measured for serogroup-specific serum bactericidal antibody (SBA) levels in the presence of baby rabbit complement (rSBA) and for IgG antibody levels. Vaccine tolerability and safety were also evaluated. For all of the measured serogroups (A, C, Y, and W-135), the meningococcal vaccine-naïve participants achieved higher postvaccination rSBA geometric mean titers (GMTs) than did those in the prior-MPSV4 group. This was statistically significant for serogroup C (512 versus 167). Percentages of participants with postvaccination titers of ≥8 and with ≥4-fold increases in prevaccination to postvaccination titers appeared to be quite similar in the 2 groups. No worrisome safety signals were detected. MCV4 induced robust immune responses and was well tolerated in Saudi Arabian children who previously received 2 doses of MPSV4 as well as in those who were previously meningococcal vaccine naïve.

Correlation between serum bactericidal activity against Neisseria meningitidis serogroups A, C, W-135 and Y measured using human versus rabbit serum as the complement source

The surrogate of protection against invasive meningococcal disease is the presence of serum bactericidal activity (SBA) at a titer ≥4 in an assay using human serum as the complement source (hSBA). However, for various practical and logistical reasons, many meningococcal vaccines in use today were licensed based on a modified SBA assay that used baby rabbit serum as the complement source (rSBA). To assess the strength of correlation between the two assay systems for serogroups A, C, W-135 and Y, we analyzed a subset of samples from adolescent subjects enrolled in a Phase II study of Novartis' MenACWY-CRM conjugate vaccine vs. an ACWY polysaccharide vaccine; samples were analyzed in parallel using hSBA and rSBA. We compared geometric mean titers (GMTs), calculated Pearson correlation coefficients between paired hSBA and rSBA results, and calculated sensitivity/specificity and likelihood ratios for an rSBA ≥8 or ≥128 for classifying hSBA ≥4, taking hSBA as the 'gold standard'. Correlations between hSBA and rSBA ranged from 0.46 to 0.78 for serogroup C, but were weaker for serogroups A, W-135 and Y (range -0.15 to 0.57). In post vaccination samples, nearly all subjects had rSBA titers ≥8, though up to 15% remained seronegative by hSBA. In post vaccination settings, rSBA titers at ≥8 or ≥128 was highly sensitive for an hSBA titer ≥4, but non-specific. In conclusion, results generated by rSBA did not accurately classify serostatus according to hSBA for serogroups A, W-135 and Y.

Meningococcal conjugate vaccines: optimizing global impact

Meningococcal conjugate vaccines have several advantages over polysaccharide vaccines, including the ability to induce greater antibody persistence, avidity, immunologic memory, and herd immunity. Since 1999, meningococcal conjugate vaccine programs have been established across the globe. Many of these vaccination programs have resulted in significant decline in meningococcal disease in several countries. Recent introduction of serogroup A conjugate vaccine in Africa offers the potential to eliminate meningococcal disease as a public health problem in Africa. However, the duration of immune response and the development of widespread herd immunity in the population remain important questions for meningococcal vaccine programs. Because of the unique epidemiology of meningococcal disease around the world, the optimal vaccination strategy for long-term disease prevention will vary by country.

Novel quadrivalent meningococcal A, C, W-135 and Y glycoconjugate vaccine for the broader protection of adolescents and adults

Meningococcal meningitis and septicemia are a persistent public health concern owing to the associated mortality and devastating long-term sequelae. People of all ages may be affected, with the disease burden being higher in at-risk groups. Vaccination is the most rational approach to the prevention of invasive meningococcal disease. A novel quadrivalent meningococcal (Men) serogroup A, C, W-135 and Y polysaccharide–protein conjugate vaccine (MenACWY-CRM), has recently been licensed for use in individuals aged at least 11 years old in the USA, Canada and Europe. One dose of MenACWY-CRM is well tolerated, and induces robust immunity to all constituent vaccine serogroups in 11–65 year old individuals. MenACWY-CRM was found to be noninferior to the quadrivalent meningococcal ACWY-diphtheria toxoid glycoconjugate vaccine, which is also licensed in the USA and Canada. In Europe, MenACWY-CRM is the first quadrivalent meningococcal glycoconjugate vaccine available to provide broader protection against Neisseria meningitidis serogroups A, C, W-135 and Y.