Statistical analyses
A total of approximately 600 participants were planned to be enrolled. Three full analysis sets (FAS) were defined: FAS1 included participants who received at least one study vaccine booster dose and had a valid Day 6 serology result; FAS2 included participants who received at least one booster dose and had a valid Day 30 serology result; and FAS3 (for antibody persistence) included participants who had a valid Day 0 serology result. Two per protocol analysis sets (PPAS) were defined: PPAS1 for Day 6, and PPAS2 for Day 30 samples, each comprising participants from FAS1 or FAS2, respectively, who had no protocol deviations. Participant data in the FAS and PPAS were analyzed according to the groups that the participants were randomized to.The safety analysis set (SafAS) was defined as all participants who received at least one dose of the study vaccine(s) and had safety data available. Safety was analyzed for participants in the SafAS according to the vaccine(s) they had received at Day 0.Immunogenicity analyses were performed in the FAS2 and PPAS2 for Day 30 results and in the FAS1 and PPAS1 for Day 6 results; antibody persistence was assessed on the FAS3. Vaccine seroresponse sufficiency (primary objective) was demonstrated if the lower limit of the 1-sided 97.5% confidence interval (CI) calculated using the Clopper–Pearson method for the percentage of participants with hSBA vaccine seroresponse against serogroups A, C, W and Y, was greater than 75%. Seroresponse sufficiency was evaluated separately for Group 1 (MenACYW-TT primed, MenACYW-TT booster) and Group 2 (MCV4-CRM primed, MenACYW-TT booster). Data from participants of the MET50 and MET43 studies were used to evaluate antibody persistence and overall trends over 3–6 years post-priming with MenACYW-YY or MCV4-CRM. Antibody titers and corresponding 95% CIs were calculated on Log10 transformed data, assuming a normal distribution for the transformed data, with antilog transformations applied to provide GMTs and their 95% CIs. Participants who were previously primed with MCV4-CRM outside of prior studies were not included in the assessment of antibody persistence.