The decline of World Health Organization standardized neutralization titres (IU/ml) post-second vaccine dose. Post-Vac2 (n = 169): sampling at about 1 month after second vaccine dose; M3 (n = 77): sampling at 3 months after second vaccine dose; M6 (M6, n = 38) sampling at 6 months after second vaccine dose. Scatterplot annotated with median and interquartile range. *Median (interquartile range). Statistical testing: as appropriate, Dunn’s multiple comparisons test and Wilcoxon rank sum test