The immunogenicity of the human papillomavirus (HPV)-16/18 AS04-adjuvanted vaccine (Cervarix®, GlaxoSmithKline Biologicals) administered according to its licensed vaccination schedule (3-dose, 3D) and formulation (20 μg of each HPV antigen; 20/20F) has previously been demonstrated. This partially-blind, controlled, randomized trial (NCT00541970) evaluated 2-dose (2D) schedules using the licensed 20/20F or an alternative formulation containing 40 μg of each antigen (40/40F), compared with the licensed 3D schedule. Healthy females stratified by age (9-14, 15-19, 20-25 y) were randomized to receive 2 doses of 20/20F at Months (M) 0,6 (n=240), 40/40F at M0,6 (n=241) or 40/40F at M0,2 (n=240), or 3 doses of 20/20F at M0,1,6 (licensed schedule/formulation, n=239). One month after the last dose, the 3D schedule was not immunologically superior to 2D schedules except in the 40/40F M0,2 group for HPV-16 (lower limit of 95% CI geometric mean antibody titer (GMT) ratio [2D/3D] < 0.5). For both HPV-16 and HPV-18, the 2D schedules in girls 9-14 y were immunologically non-inferior to the 3D schedule in women 15-25 y (the age group in which efficacy has been demonstrated) (upper limit of 95% CI for GMT ratio [3D/2D] < 2) one month after the last dose. At Month 24, non-inferiority was maintained for the 2D M0,6 schedules in girls 9-14 y versus the 3D schedule in women 15-25 y. All formulations had acceptable reactogenicity and safety profiles. These results indicate that the HPV-16/18 vaccine on a 2D M0,6 schedule is immunogenic and generally well tolerated in girls 9-14 y and that the 2D schedule is likely adequate for younger females.

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Generalizability of findings is not assured even if internal validity of a research study is addressed effectively through design. Strict controls to ensure internal validity can compromise generalizability. Researchers can and should use a variety of strategies to address issues of external validity and enhance generalizability of findings. Enhanced external validity and assessment of generalizability of findings can facilitate more appropriate use of research findings.

This randomized, partially-blind study (ClinicalTrials.gov registration number NCT00541970) evaluated the immunogenicity and safety of 2-dose (2D) schedules of the HPV-16/18 AS04-adjuvanted vaccine. Results to month (M) 24 have been reported previously and we now report data to M48 focusing on the licensed vaccine formulation (20 μg each of HPV-16 and -18 antigens) administered at M0,6 compared with the standard 3-dose (3D) schedule (M0,1,6). Healthy females (age stratified: 9–14, 15–19, 20–25 years) were randomized to receive 2D at M0,6 (n = 240) or 3D at M0,1,6 (n = 239). In the according-to-protocol immunogenicity cohort, all initially seronegative subjects seroconverted for HPV-16 and -18 antibodies and remained seropositive up to M48. For both HPV-16 and -18, geometric mean antibody titer (GMT) ratios (3D schedule in women aged 15–25 years divided by 2D schedule in girls aged 9–14 years) at M36 and M48 were close to 1, as they were at M7 when non-inferiority was demonstrated. The kinetics of HPV-16, -18, -31, and -45 antibody responses were similar for both groups and HPV-16 and -18 GMTs were substantially higher than natural infection titers. The vaccine had a clinically acceptable safety profile in both groups. In summary, antibody responses to a 2D M0,6 schedule of the licensed vaccine formulation in girls aged 9–14 years appeared comparable to the standard 3D schedule in women aged 15–25 years up to 4 years after first vaccination. A 2D schedule could facilitate implementation of HPV vaccination programs and improve vaccine coverage and series completion rates.