Statistical analysis
Variables are reported as means +/− standard deviations, medians and interquartile range (IQRs) or CI95%, and numbers and percentages. Immunogenicity response was assessed by means of nAb geometric mean titers (GMTs), and seroconversion rates. Fourfold seroconversion (4×-seroconversion) or tenfold seroconversion (10×-seroconversion) were defined respectively as an increase in neutralizing antibodies equal or higher than four- or ten-times when the baseline nAb titers before the booster vaccine were detectable or four times the lower detection limit when the baseline concentration was not detectable. Differences in mean, geometric mean, or percentage values between groups and among prior vaccine platforms were assessed by means of Mann Whitney u test, Wilcoxon pair-matched test, Kruskal-Wallis test with post hoc Dunn’s for multiple comparisons, Fisher exact test, or Chi-square distribution, as appropriate. 95% CI were calculated using the Wilson/Brown method. Missing data were assumed to be missing at random. Statistical analyses were done using GraphPad Prism v8.4.2 (GraphPad Software, San Diego, CA), Two-sided 