http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5602a1.htm?s_cid=rr5602a1_xVaccination of females who were seropositive to a specific vaccine HPV type at enrollment resulted in higher antibody titers to that type, particularly after the first dose, compared with those seronegative at enrollment (109), suggesting a boosting of naturally acquired antibody by vaccination. In studies among females aged 16--26 years, the interval between the first and second dose of vaccine ranged from 6 to 12 weeks. Variation in the interval did not diminish the GMTs post-vaccination. Likewise, little impact of intervals was observed between the second and third dose ranging from 12 to 23 weeks.