Evaluating MenB Vaccines Beyond the Traditional hSBA Assay

Evaluating MenB Vaccines Beyond the Traditional hSBA Assay

In an April 2025 mSphere minireview, researchers outlined the evolving methodologies used to evaluate the performance of multicomponent serogroup B meningococcal (MenB) vaccines, such as 4CMenB and MenB-FHbp.

Developed to prevent invasive meningococcal disease (IMD), these vaccines were licensed based on serological surrogate markers, as conventional efficacy trials were infeasible due to the low incidence of IMD. Among these surrogates, the human serum bactericidal antibody (hSBA) assay has played a central role, measuring vaccine-induced immune responses using a limited set of antigen-specific test strains and exogenous human complement.

While accepted for regulatory purposes, the hSBA assay offers a narrow view of vaccine performance compared to real-world effectiveness data. Its capacity to capture protection across the antigenic diversity of circulating MenB strains is limited, prompting the development of additional tools that extend beyond this foundational method.

Strain Coverage: A Moving Target
Surface protein variability among MenB isolates complicates the assessment of vaccine effectiveness. To address this, several predictive systems have been developed to estimate the proportion of circulating strains potentially covered by multicomponent vaccines. These include:

• MATS (meningococcal antigen typing system), which uses ELISA-based quantification of key vaccine antigens (fHbp, NadA, and NHBA) combined with PorA genotyping to estimate strain coverage
• gMATS (genetic MATS), a DNA sequence-based extension of MATS that enables prediction using genomic data, even when cultured isolates are unavailable
• MEASURE (meningococcal antigen surface expression assay), a flow cytometry method that quantifies expression levels of factor H binding protein (fHbp) on the bacterial surface but does not account for antigenic variability
• MenDeVAR (meningococcal deduced vaccine antigen reactivity) index, which integrates genomic and serologic data to assess whether a strain’s antigens are likely to be matched or cross-reactive with vaccine components

MATS-based predictions for 4CMenB range from 66 to 91 percent, but these estimates may be conservative because they don’t account for antigen synergy or unmeasured components like PorB and OpcA.

For example, in England and Wales, MATS and gMATS predicted coverage of 66–73 percent and 72–73 percent, respectively, for a panel of invasive strains, whereas hSBA testing indicated 88 percent killing activity for the same set.

A New Assay Reflecting Real-World Conditions
The endogenous complement hSBA (enc-hSBA) assay was developed to evaluate bactericidal activity under conditions that better simulate those encountered in vivo. Unlike traditional hSBA, this method preserves each vaccine recipient’s native complement system and uses a diverse panel of 110 MenB strains selected to represent nearly 90 percent of global circulating isolates.

In a pivotal phase 3 study, two doses of 4CMenB administered two months apart resulted in test-based strain coverage of 78.7 percent and responder-based coverage of 84.8 percent, indicating that most vaccinated individuals mounted a bactericidal response against a majority of strains.

A longer, six-month interval between doses yielded slightly higher estimates. The MenABCWY vaccine, which combines 4CMenB and MenACWY components, showed comparable performance: 77.9 percent test-based and 84.1 percent responder-based coverage.

PorA: A Cautionary Outlier
While immunogenic, the PorA antigen is highly variable. In the same trial, traditional hSBA failed to demonstrate non-inferiority for PorA-specific responses.

However, the enc-hSBA assay showed consistent bactericidal activity against all eight strains expressing the PorA P1.4 subtype. This highlights the added value of the enc-hSBA assay in detecting functional immune responses that may not be evident with antigen-specific assays alone.

From Bench to Population
The enc-hSBA results align closely with real-world vaccine effectiveness. A mass adolescent immunization program in South Australia showed a 78.5 percent reduction in MenB IMD cases over three years; this is comparable to the responder-based coverage reported in the trial.

Across multiple regions, observed vaccine effectiveness of 4CMenB has ranged from 71 to 100 percent, which is consistent with enc-hSBA-derived estimates.

Toward a Complementary Toolkit
Each method used to evaluate MenB vaccines contributes a distinct perspective to understanding vaccine performance. The traditional hSBA assay remains essential in early development and regulatory review, offering quantitative immunogenicity endpoints. Predictive tools such as MATS, gMATS, and the MenDeVAR index are best suited for estimating population-level strain coverage and informing post-licensure surveillance.

The enc-hSBA assay adds value in clinical trials by assessing immune breadth against a genetically diverse panel of MenB strains under physiologically relevant conditions. It also detects synergistic antibody activity across multiple antigens and yields both test-based and responder-based metrics, providing a broader view of functional immune response that aligns with real-world protection.

Looking Ahead
While not a replacement for existing assays, the enc-hSBA provides complementary insights that enhance the overall assessment of multicomponent MenB vaccines. As evidence accumulates, this assay may help bridge the gap between immunogenicity data and observed vaccine effectiveness, particularly for emerging vaccine formulations such as MenABCWY.

Reference:

1. Borrow R, Tomasi Cont L, Toneatto D, et al. Methods to evaluate the performance of a multicomponent meningococcal serogroup B vaccine. mSphere. 2025;10(4):e0089824. doi:10.1128/msphere.00898-24