Investigate the durability of vaccine efficacy (VE) against HPV16/18 infections and antibody response among non-randomized women who received a single-dose of the bivalent HPV vaccine compared to women who received multiple doses and unvaccinated women.
HPV infections were compared between HPV16/18 vaccinated women aged 18 to 25 years who received one (N = 112), two (N = 62), or three (N = 1365) doses, and age- and geography-matched unvaccinated women (N = 1783) in the long-term followup of the Costa Rica HPV Vaccine Trial. Cervical HPV infections were measured at two study visits, approximately 9- and 11-years after initial HPV vaccination, using NCI next-generation sequencing TypeSeq1 assay. VE and 95% confidence intervals (CI) were estimated. HPV16/18-antibody levels were measured on all one- and two-dose women, and a subset of three-dose women, using a virus-like particle-based ELISA (n = 448).
Median follow-up for the HPV-vaccinated group was 11.3 years (interquartile range [IQR]: 10.9-11.7 years) and did not vary by dose group. VE against prevalent HPV16/18 infection was 80.2% (95%CI 70.7-87.0%) among three-dose, 83.8% (95%CI 19.5-99.2%) among two-dose, and 82.1% (95%CI 40.2-97.0%) among single-dose women. HPV16/18 antibody levels did not qualitatively decline between years four and 11, regardless of the number of doses given, although one-dose titers continue to be statistically significantly lower compared to two- and three-doses.
More than a decade after HPV vaccination, single-dose VE against HPV16/18 infection remained high and HPV16/18 antibodies remained stable. A single dose of bivalent HPV vaccine may induce sufficiently durable protection that obviates the need for more doses.

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