Study population

Out of the 5570 invitations sent to eligible older adults, 512 individuals (9.5%) expressed interest in participating in the study and 223 participants were included to receive a primary MenACWY-TT dose (Fig. 1). Following the loss of one participant before the first post-vaccination sampling, analyses were performed on a cohort of 222 participants (median age 73 years; IQR: 69–76; 42.8% female; see Supplementary Table 2 for detailed baseline characteristics). One year after primary vaccination, a subset of 104 participants (baseline median age 73 years; IQR: 69–77; 44.2% female; Fig. 1, and Supplementary Table 2) received a booster MenACWY-TT dose.

This diagram outlines participant recruitment, the timing of vaccination and the different post-vaccination timepoints. Upon enrollment, participants were randomly assigned to receive a booster vaccination one year after primary immunization (1 y) or not.

MenACWY-specific rSBA responses following primary MenACWY-TT vaccination

At baseline, proportions of participants with rSBA titers ≥8 varied per serogroup, ranging from 12.2% for MenY to 23.9% for MenA (Fig. 2B and Table 1). One month following vaccination, rSBA titers significantly increased for all four serogroups compared to baseline (all p < 0.0001; Fig. 2A and Table 1). Additionally, 96% (MenA), 99% (MenC), 90% (MenW), and 92% (MenY) of older adults achieved protective titers (rSBA ≥8), and 90% (MenA), 86% (MenC), 88% (MenW), and 90% (MenY) of older adults reached rSBA titers of ≥128 (Fig. 2B and Table 1). One year post-vaccination, rSBA titers and the proportions of participants with rSBA titers ≥8 and ≥128 remained significantly higher than baseline (all p < 0.0001), although a significant decline was observed compared to one month post-vaccination (all p < 0.0001; Fig. 2 and Table 1). At one year post-vaccination, 75% (MenA), 82% (MenC), 78% (MenW), and 75% (MenY) of participants had rSBA titers ≥8, while 54% (MenA), 56% (MenC), 73% (MenW), and 63% (MenY) maintained titers ≥128 (Fig. 2B, Table 1). At two-year post-vaccination, rSBA titers were still significantly higher than baseline (p < 0.0001) in primary-only vaccinees (n = 110; Fig. 2A and Table 1). Of these participants, 73% (MenA), 74% (MenC), 62% (MenW), and 58% (MenY) still had rSBA titers ≥8. Although no effect of age on rSBA responses was observed, MenA- and MenC-specific rSBA titers were significantly higher in females than in males (p = 0.014 and p < 0.001, respectively, one-month post vaccination; Supplementary Table 2).

MenACWY-specific rSBA responses A and corresponding percentages of participants with rSBA titers ≥8 and ≥128 (B), ps-specific IgG (C) and IgM (D) concentrations were determined pre-vaccination (pre-), at one month (1 m) and one year (1 y) post-primary MenACWY-TT vaccination for all participants (n = 222). At two years (2 y) post-primary vaccination, responses were assessed only in participants who did not receive a booster vaccination at 1 y (n = 110). Geometric mean titers (GMTs) and geometric mean concentrations (GMCs) are indicated in black, with error bars representing the 95% confidence intervals. Following log10-transformation, rSBA and antibody responses were compared between timepoints using paired generalized linear mixed models (GLMM), followed by post-hoc Emmeans tests and subsequent Bonferroni correction for multiple testing. Proportions of participants with rSBA ≥8 or ≥128 (with 95% CI) were calculated with the Wilson score interval with continuity correction.

MenACWY ps-specific IgG and IgM responses following primary MenACWY-TT vaccination

At one month post-vaccination, significant increases in ps-specific IgG and IgM concentrations were observed compared to baseline for all serogroups (all p < 0.0001), which remained elevated for two years (all p < 0.0001; Fig. 2C, D and Table 1).

Interestingly, while MenA- and MenC-specific IgG and MenACWY-specific IgM concentrations peaked at one month, followed by a decline at one year post-vaccination (all p < 0.0001), MenW-specific IgG concentrations increased significantly from one month to one year post-vaccination (p < 0.0001; Fig. 2C, D, and Table 1). Moreover, no significant change in MenY-specific IgG concentrations was observed between these timepoints. Upon further analysis, a substantial number of participants demonstrated a ‘slow’ IgG response, with higher IgG concentrations at one year compared to one month post-vaccination for MenW (n = 143) and MenY (n = 98; Supplementary Fig. 1). Interestingly, while these individuals had significantly lower IgG concentrations at one month post-vaccination compared to those exhibiting the expected antibody peak at one month (p < 0.0001 for both serogroups), IgG concentrations were similar at one year post-vaccination (Supplementary Fig. 1). A similar slow increase in rSBA or IgM responses was observed in only small numbers of participants.

MenACWY-specific rSBA responses following booster MenACWY-TT vaccination

In the booster cohort (n = 104), significant increases in rSBA titers were observed at one month post-booster vaccination compared to pre-booster titers for all serogroups (all p < 0.0001; Fig. 3A and Table 2). Additionally, proportions of older adults with rSBA titers ≥8 increased for MenA (98%), MenC (95%), MenW (98%), and MenY (94%), though not significant for MenC (p = 0.068; Fig. 3B and Table 2), whereas proportions with titers ≥128 increased for all serogroups (p < 0.01; Fig. 3B and Table 2). One year post-booster vaccination, MenACWY-specific rSBA titers declined significantly compared to one month post-booster (p < 0.0001; Fig. 3A and Table 2). Interestingly, no significant differences were found between pre-booster and one year post-booster vaccination rSBA titers, except for an increase for MenA (p < 0.0001; Fig. 3A and Table 2). One month post-booster vaccination rSBA titers for MenA (p = 0.0033) and MenC (p < 0.0001) were lower, and those for MenW and MenY were comparable compared to one month post-primary vaccination. No effect of age or sex was observed on post-booster vaccination rSBA responses (Supplementary Table 2). Two years post-primary vaccination, rSBA titers in the booster cohort were significantly higher than those observed in the primary-only cohort for MenA (p < 0.0001), MenW (p = 0.0002), and MenY (p = 0.0030), but not for MenC (Supplementary Fig. 2A).

Data shown are for the booster cohort (n = 104), which includes only participants who received a MenACWY-TT booster vaccination at one year (1 y) post-primary vaccination. MenACWY-specific rSBA responses (A), corresponding percentages of participants with rSBA titers ≥8 and ≥128 (B), ps-specific IgG (C), and IgM (D) concentrations were assessed pre-booster (1 y), at one month (1 y + 1 m), and one year (1 y + 1 y) following booster vaccination. For reference, responses measured before and after primary vaccination in booster cohort participants are also shown. Geometric mean titers (GMTs) and geometric mean concentrations (GMCs) are shown in black, with error bars representing 95% confidence intervals. Following log10-transformation, rSBA and antibody responses were compared between timepoints using paired generalized linear mixed models (GLMM), followed by post-hoc Emmeans tests and subsequent Bonferroni correction for multiple testing. While statistical comparisons were performed across all timepoints, the significant p-values are only shown for the following comparisons: 1 y vs. 1 y + 1 m, 1 y vs. 1 y + 1 y, 1 m vs. 1 y + 1 m, 1 y + 1 m vs. 1 y + 1 y. Proportions of participants with rSBA ≥8 or ≥128 (with 95% CI) were calculated with the Wilson score interval with continuity correction.

MenACWY ps-specific IgG and IgM responses following booster MenACWY-TT vaccination

One month post-booster vaccination, ps-specific IgG concentrations significantly increased for all serogroups and remained significantly higher at one year (for MenA, MenW, and MenY) compared to pre-booster concentrations (p < 0.0001; Fig. 3C and Table 2). Nevertheless, a significant decline in IgG was noted between one month and one year post-booster vaccination for all serogroups (p < 0.0001). Notably, IgG concentrations were significantly higher one month post-booster vaccination compared to one month post-primary vaccination for MenA, MenW, and MenY (p < 0.01), but not for MenC (p = 0.76).

Ps-specific IgM concentrations increased significantly one month post-booster compared to pre-booster vaccination (p < 0.0001; Fig. 3D and Table 2). However, for MenC, MenW, and MenY, IgM returned to pre-booster concentrations by one year. Additionally, IgM concentrations at one month post-booster vaccination did not exceed those observed at one month post-primary vaccination.

Two years after primary vaccination, MenACWY ps-specific IgG and MenA ps-specific IgM concentrations were significantly higher in the booster cohort compared to the primary-only cohort (all p < 0.05; Supplementary Fig. 2B, C).

Correlations between MenACWY-specific rSBA, IgG, and IgM responses following primary and booster MenACWY-TT vaccination

Correlations between ps-specific IgG and IgM concentrations were either absent or weak across all serogroups and timepoints following primary as well as booster MenACWY-TT vaccination (Fig. 4A, B).

The Spearman correlation coefficient and corresponding levels of significance are shown for each pairwise combination of MenACWY-specific rSBA titers, IgG levels, and IgM levels for the whole cohort (n = 222) following primary vaccination (A). At two years (2 y) post-primary vaccination, responses were only assessed in participants who did not receive a booster vaccination at 1 y (n = 110). Additionally, correlations following booster vaccination are shown for the booster cohort (n = 104) (B), which includes only participants who received a MenACWY-TT booster vaccination at one year (1 y) post-primary vaccination. For reference, correlations observed before and after primary vaccination are also shown for the booster cohort, to allow for direct comparison between timepoints. Correction for multiple testing was performed using the Bonferroni method. The strength of the correlation is depicted by color intensity, while the significance level is indicated by the size of the dot. Spearman rho values are displayed for each pairwise correlation.

Following primary vaccination, correlations between rSBA titers and IgG concentrations were moderate for MenA and MenC, and weak for MenW and MenY (p < 0.001; Fig. 4A). Interestingly, strong correlations were found between post-primary vaccination MenW- and MenY-specific rSBA titers and IgM concentrations (p < 0.001; Fig. 4A). Besides, these correlations were moderate for MenA and MenC (p < 0.001). Overall, one month post-primary vaccination rSBA titers showed stronger correlations with ps-specific IgM than with IgG for all serogroups.

One month post-booster vaccination, rSBA titers correlated stronger with ps-specific IgM than with IgG for MenC and MenY (Fig. 4B). For MenA and MenC, these correlations were moderate-to-strong for both IgG and IgM. Notably, MenW and MenY-specific rSBA titers demonstrated stronger correlations with IgG at one month post-booster vaccination, than correlations observed at one month post-primary vaccination (Fig. 4B).

The role of MenACWY ps-specific IgM in conferring bactericidal activity

To assess the relative importance of ps-specific IgG and IgM responses in conferring bactericidal activity, individuals were ranked high for IgG and/or IgM at one-month post-primary vaccination (Fig. 5A). As such, four groups were allocated: both high, both low, high IgG-low IgM and vice versa. MenW and MenY-specific rSBA titers were significantly higher in participants with low ps-specific IgG but high IgM concentrations, as compared to participants with high IgG but low IgM concentrations (p < 0.0001 for both; Fig. 5A). Nevertheless, this was not observed for MenA and MenC.

Participants were ranked high or low for IgG and/or IgM concentrations at one-month post-primary vaccination. As such, four groups were assigned: both high, both low, high IgG-low IgM and vice versa. Based on these groups, rSBA titers at one month post-primary vaccination were compared between the different Ig-profile groups (A) using the Kruskal-Wallis test, followed by Dunn’s post-hoc test and Bonferroni correction for multiple testing. The effect of IgG or IgM depletion on post-vaccination rSBA titers were determined in a subset of one-month post-vaccination serum samples (n = 28). MenC- (B) and MenW- (C) specific IgG concentrations (left) and rSBA titers (right) following IgG depletion. MenC- (D) and MenW- (E) specific IgM concentrations (left) and rSBA titers (right) following IgM depletion. Original and depleted samples were compared using the Wilcoxon matched-pairs signed rank test.

Lastly, we depleted either IgG or IgM from a subset of post-vaccination serum samples (n = 28) and determined MenC and MenW-specific rSBA titers (Fig. 5B–E). Upon IgM depletion, significant decreases in rSBA titers were found in depleted versus original sera (MenC: p = 0.0008; MenW: p < 0.0001, Fig. 5D, E), but not following IgG depletion (MenC: p = 0.49; MenW: p = 0.37, Fig. 5B, C).