Maintenance of HPV-16 and HPV-18 specific IgG. (A) Frequency of study subjects with detectable binding HPV-16 and HPV-18 specific IgG. (B) HPV-16 and HPV-18 specific IgG titers measured by ELISA shown as box and whiskers plots. Statistical comparisons were made using Fisher’s exact probability test (A) and Mann-Whitney test (B), ** P < 0.01, *** P < 0.001; for HPV-16 10–12 y, n = 64 (Cervarix) and n = 31 (Gardasil); for HPV-16 16–20 y, n = 34 (Cervarix) and n = 20 (Gardasil); for HPV-18 10–12 y, n = 60 (Cervarix) and n = 31 (Gardasil); for HPV-18 16–20 y, n = 33 (Cervarix), and n = 20 (Gardasil).