The study involved 18,174 women aged 16-24, who were randomly assigned to receive a quadrivalent HPV 6/11/16/18 vaccine or a placebo. Injections were given at baseline and two and six months later, and the women were followed up for an average of three years.
Among women who were naive to HPV 16 and HPV 18, the vaccine was 100 per cent effective against lesions associated with the viral strains. But this fell to 71 per cent in those previously exposed to HPV. In an intention-to-treat analysis the vaccine was associated with a 49 per cent reduction in high-grade vulval intraepithelial neoplasia and vaginal intraepithelial neoplasia. This reduced risk was found irrespective of whether HPV DNA was found in the lesions.
Lancet 2007; 369: 1693-1702