In initially seronegative participants , anti-HPV-16 and -18 antibody geometric mean titers were 65.8 - and 33.0-fold higher than those associated with natural infection ( NCT00122681 ) and 5.0 - and 2.5-fold higher than those measured at month 69-74 in a study demonstrating vaccine efficacy in women aged 15-25 years ( NCT00120848 ) .