The magnitude of the plasmablast response also inversely correlates with preexisting antibody (Ab) titers. A, Absolute numbers of plasmablasts in blood at 1 week and 1 month postvaccination (M6W1 and M7) were normalized to numbers at M6, when subjects received their third quadrivalent human papillomavirus (4HPV) vaccine dose. Likewise, numbers of plasmablasts at M24W1 and M25 were normalized to numbers at M24, when subjects received a fourth booster 4HPV dose. B, Changes in the numbers of plasmablasts from M6 to M6W1 in response to the third dose at M6 (black dots) or from M24 to M24W1 in response to the booster dose at M24 (pink dots) inversely correlated with the preexisting binding anti–HPV-16 immunoglobulin G (IgG) titers at the time of vaccination. Pearson correlation. n = 4 human subjects.