||Herrero R, Wacholder S, Rodríguez AC, Solomon D, González P, Kreimer AR, Porras C, Schussler J, Jiménez S, Sherman ME, Quint W, Schiller JT, Lowy DR, Schiffman M, Hildesheim A, Costa Rica Vaccine Trial Group, Alfaro M, Barrantes M, Bratti M, Cárdenas F, Cortés B, Espinoza A, González P, Guillén D, Herrero R, Jiménez SE, Morales J, Morera LA, Pérez E, Porras C, Rodríguez AC, Rivas L, Villegas L, Atmella I, Bonilla J, Freer E, García-Piñeres A, Ramírez M, Silva S, Hildesheim A, Kreimer AR, Lowy DR, Macklin N, Schiffman M, Schiller JT, Sherman M, Solomon D, Wacholder S, Pinto L, Kemp T, Eklund C, Hutchinson M, Sidawy M, Quint W, van Doorn LJ
||Target groups for human papillomavirus (HPV) vaccination are controversial. We evaluated vaccine efficacy (VE) against 1-year persistent infection, stratified by age and sexual behavior, among young women in Costa Rica. We randomized 7,466 healthy women 18 to 25 years of age to HPV16/18 or hepatitis A vaccine (follow-up, 50.4 months). According-to-protocol (ATP) cohorts included compliant HPV-negative women; intention-to-treat (ITT) included all randomized women. ATP VE was 90.9% (95% CI, 82.0-95.9) against HPV16/18 infections, 44.5% against HPV31/33/45 (95% CI, 17.5-63.1), and 12.4% (95% CI, -3.2 to 25.6) against any oncogenic infection. Overall ITT VE against HPV16/18 infections was 49.0%, but ATP and ITT VE almost reached 100% in year 4 of follow-up. ATP efficacy against HPV16/18 was similar by age, but ITT VE was greatest among youngest women (68.9% among those 18-19 years of age; 21.8% among those 24-25 years of age) and 79.8% among virgins. Among previously unexposed women, vaccination is highly efficacious against HPV16/18 and partially against HPV31/33/45. Vaccination is most effective in women and girls before they initiate sexual activity, with programmatic and individual decision implications.